Archive for the 'Tower Defense' Category


Orcs Must Die!: Story and Character

Just as the UI in Orcs Must Die! manages to get along without a lot of explanation, so too does the story. You start off in the middle of an emergency that doesn’t require world-building to be intelligible, and most of what you learn about your situation later on comes from offhand comments rather than cutscenes and similar infodumps. The story’s major turn, the revelation that the orcs are being organized by some external force, is foreshadowed by some of the orcs’ random shouts: in addition to expected cries of “Kill the humans!” and such, they occasionally say things like “Yes, mistress!” and “Get out of my head!”. Just as well — when the game does finally does start to provide exposition in the form of psychic dialogue between the PC and the sorceress who’s driving the horde, it’s often drowned out by the background music and the clamor of battle.

Now, the game’s formal properties impose certain things on the story. You’re playing a character who physically exists in the gameworld, who has a location and and has to run around tending to emergencies wherever they crop up. At the same time, you’re basically acting alone. There are guardians and weavers, sure, but you’re the only war mage, and that means you have to both set up and execute the defense of every route to the rift in each and every fortress, even the ones that are clearly set up to be optimally guarded by teams of two or four people. Why it is like this? Because, we learn, all the other war mages who would normally be helping out are already dead at the hands of the orcs. There are a few different ways you could take this story. The creators of this game decided to make it a comedy.

The main way it does this is by making the PC into the war mage least suited for this challenge: young, brash, wisecracking, disrespectful about his recently-dead mentor, and above all, conceited. The sort of person who says “Booyah!” after a kill. It’s the sort of character Nolan North is known for, with a little more fratboy mixed in. In short, he’s a jerk, and the story is the story of a jerk justifying his jerkitude by triumphing where no one else expected he could. Given this, I suppose it’s fitting that the true antagonist is a controlling, manipulative woman. It’s all part of the PC’s world view. We never encounter the sorceress directly — in the end, you defeat her simply by closing the rifts and trapping her in the orcs’ homeworld — but we see her in cutscenes, and she’s exactly the sort of hooker-booted hottie that the PC would go for were she not so haughty and domineering. The orcs who do her bidding are of course exclusively male, the mind control magic being a metaphor for feminine wiles. The PC is of course immune to this magic, being too full of himself to let anyone else in. No woman can tame him. He isn’t just the jerk triumphant, he’s triumphant because he’s a jerk.

I feel like this is a sort of hero that’s been becoming more popular in games lately, which makes me a little worried about the zeitgeist. Games are really good at provoking identification with the protagonist. Is this really the sort of person we want to identify with?

Orcs Must Die

The first round of holiday sales is underway, leaving me scrambling to finish up some games to make room on the Stack for new stuff. Orcs Must Die is in fact among my new acquisitions, but looked like it would probably be quick to finish, due to its structural resemblance to Plants vs Zombies: not only is it essentially a tower defense game, it pulls the same trick of introducing one new game element per level, which means the game ends once it’s reached maximal complexity. And indeed, in a single day of obsessive play, I’m managed to complete every level but the last. So expect a second and final post tomorrow.

I say it’s essentially a tower defense, but it’s really a hybrid of tower defense and shooter. The whole idea is that in each level you’re trying to prevent hordes of orcs and related monsters (kobolds as swarmers, ogres as bosses) from reaching a dimensional rift, which is in the middle of a fortress presumably built around it for the specific purpose of keeping the orcs away. This is a fully 3D structure, and you have an avatar inside it. You can place various sorts of traps on the walls, floors, and ceiling, as well as summon “guardians” who fight with sword or bow, but you can also fight the orcs directly, with a repeating crossbow, bladestaff, and various spells that you acquire over the course of the game. And in fact you pretty much have to do both, picking off orcs manually when they survive the traps. Starting at level 11, you can buy enhancements of various sorts from “weavers”, but you have to choose between “steel weavers”, who enhance your traps and guardians (for example, making the traps reset faster or giving the archers flaming arrows), or “elemental weavers”, who enhance your personal combat abilities (increasing your health, making spells do more damage). I personally want to play this more as a tower defense game than as a shooter, so I’ve pretty much always taken the steel weaver — at least, until the knowledge weavers became available at level 19, with their tempting treats like making the rift itself produce lightning bolts, or occasionally reanimating dead orcs to fight on your side.

But even treating the game as a tower defense, it’s a peculiar tower defense, due to the fact that you’re seeing the whole thing from inside. (Shades of Intelligent Qube!) The game helps minimize this limitation by granting you a great deal of mobility: your traps don’t affect you at all, you can jump off balconies and over any barricades you’ve placed, and there are often teleport gates joining distant parts of the stronghold. (It took me a while to realize that the orcs couldn’t go through the gates. I wasted some cash in the early levels barricading them.) But it offsets this by making you vulnerable. There are types of occasional enemy that ignore the rift, choosing instead to attack you and any guardians you’ve summoned — and for that reason alone, it’s important to have a few guardians around as distractions. For that matter, ordinary orcs will sometimes decide to chase you if you’re close enough, which means that by your presence you can distract them from the rift.

In short, for all its focus on a single sort of dungeon encounter, this is a pretty rich game. The thing that really impresses me, though, is the UI design. Placing objects in three dimensions is a nontrivial task, and there’s basically no explanation, documentation, or tutorial here, other than a few on-screen prompts, such as “Press R to rotate”. And yet it all just works. You choose a trap to place in the same way as you choose a weapon, and you also aim it like a weapon at the surfaces that can support it. When you’re aiming at a valid spot, the trap appears as a transparent model, with, if relevant, another transparency indicating its area of effect, so you know if that arrow trap reaches all the way across the hallway or not. Outside of trap placement, there are a number of little touches like the targeting reticule for the crossbow that widens if you fire rapidly, clearly indicating without words that your aim is becoming less accurate. Perhaps this is stuff that you need to already be familiar with games to understand, but it works for me.

Defense Grid: Final Ramblilngs

Well, I’ve managed to squeeze my way through the last level of Defense Grid‘s campaign mode. The game content never acknowledges my speculations from the previous post, treating victory as simply victory and the salvation of the planet. But I suppose that if every game had a subversive twist, it would stop being subversive. Something has to play it straight, and Defense Grid is a straighter game than most I play.

So let’s just comment on a few more points of mechanical interest. In fact, let’s start with interest itself, one of the game’s major experiments that I neglected to describe in my previous post. Any money you keep in reserve compounds interest at a rate determined by the number of power cores at your base. Obviously this provides an extra motivation to protect the cores, but it also gives you a reason to spend as little as you can get away with in the early part of the level, and these motivations are in tension. The net effect is a desire for maximal efficiency in situations where other tower defense games would have you just trying to overpower stuff by building as fast as possible.

My own experience is that the need to build stuff generally wins out. This is especially true in the final few levels, which are large enough that the space always seems underutilized. The temptation to build everywhere is strong, but should be resisted, if my narrowly-successful strategies are at all indicative of the correct approach. Building level-1 towers everywhere just takes away money you could be spending on upgrading existing towers, which seems to usually be the more efficient approach; that is, in most cases, upgrading a tower increases its damage potential more than an equal cost of additional towers would, especially if it’s wisely placed. Note that there’s still often good reason to refrain from upgrading, because upgrading isn’t instant. In fact, it’s agonizingly slow, and while it’s in progress, the tower undergoing the upgrade is inactive. So you always need enough active towers to pick up the slack when you get the cash to upgrade something, and this guarantees that you won’t be able to get the cash to upgrade it as soon as you want to.

Mind you, if you’re frequently spending all your money, and therefore not earning interest on it, you don’t really need the power cores to be at your base all the time. And, in fact, having them scattered along the path can at times be a boon in disguise. You see, there are occasional enemies that fly. They don’t follow the same path as the ground units, but follow their own swooping flight plan, unobstructed by your towers. If they manage to fly their entire path without getting shot down, they’ll grab a power core from your base and immediately take off into the heavens, without giving you a chance to get it back. But if all your power cores are elsewhere at that point, they just leave. There were points when I was sure I was doomed, because I had only one or two power cores left and couldn’t get my missile bases operational fast enough to wipe out oncoming fliers, only to be saved when a ground unit grabbed the power cores first.

Incedentally, there are only three levels of tower — that is, you can only upgrade towers twice. It turns out this is enough to be satisfying. There are really only three conceptual slots for an upgrade system anyway: you’ve got things that you haven’t upgraded at all, things you’ve upgraded some but can still upgrade further, and things that are at max level and can’t be upgraded further. Furthermore, three is few enough that the game can assign a vividly distinct color to each upgrade level — specifically, green, yellow, and red — thereby making the level of everything on the screen immediately apparent. Aliens use the same color scheme, and therefore presumably also come in only three strengths per type. I just wish that the types of tower had something like the same level of clarity! Most of them just look like towers in slightly different shapes. I sometimes lose track of which tower is which type and wind up hurriedly upgrading the wrong things.

Defense Grid: Dead World

The environments in Defense Grid: The Awakening are all brown and barren, rocky deserts with old and crumbling structures on them. There isn’t a lot of in-game information about the world and its history, but you know that the “aliens” have attacked in the past — long enough ago for the defense grid to need awakening. The sole speaking role belongs to an AI, the uploaded mind of a human who was involved in the defense the last time. He talks like a stereotypical British colonial officer and enthuses about how beautiful things used to be. At times he gets confused and addresses you by the name of his dead son, who he failed to protect during the first invasion. In other words, there’s every sign that the human presence is long wiped out, and this is a dead planet, with nothing worth defending.

This would be a familiar twist from a certain other tower defense game, but I don’t think that’s the only reason I think of it. I can believe I’m unduly influenced by the degree to which this game feels like a RTS game, though. All tower defense games are of course descendants of the RTS, but I’m talking here mostly about superficial matters like the mere presence of voice acting and the measured pace at which it introduces new elements. (For a while, it felt like the entire game, like the single-player campaign in many a RTS, was an extended tutorial.) In a typical Warcraft/Command & Conquer-influenced RTS, your base is a hive of activity, with autonomous worker units harvesting resources and repairing buildings, but here, it’s just a repository for power cores. So even the things you’re defending aren’t alive (even if they are the only things on your side capable of moving under their own power).

It all makes me speculate that this scenario is the most natural fit for a tower defense, this defense of the dead from the living (which would make Plants vs Zombies a clever inversion). It’s part of the genre’s definition that the enemy is active and your tools are passive, waiting for something to kill. Perhaps you could make a satisfying tower defense set in an Egyptian tomb, placing curses to foil looters and acheologists.

But then, I seem to be a bit obsessed with finding themes relating to death and mortality in games recently, so take it as you will.

Defense Grid: The Awakening

So, I bought a couple new Steam indie bundles recently. (They’re calling them “bundles” now. They used to call them “packs”. I’m guessing there’s a perfectly humble reason for this change in terminology.) And one of them contained Defense Grid, another of Steam’s perennial discount items that I’ve somehow managed to avoid purchasing until now. I’d been curious about it, however, because it seemed to be the first tower defense game with A-list production values — by which, admittedly, I mainly just mean 3D models and voice acting. But that’s a somewhat less rare combination today than three years ago when it was released. Having played it most of the way through now, I have to say that it’s pretty by-the-book, its basic gameplay not much different from Desktop Tower Defense and its myriad online imitators. Things come along a path, you place towers to kill them, and in the process you earn money that you use to build more towers or upgrade existing ones. But it does do a few interesting things that I think are worth pointing out.

For starters, there’s the control scheme. You have a cursor in the center of the screen. Move your mouse and the cursor stays put while the rest of the world moves. This is, of course, basically how first-person shooters work, but you’re not rotating in place here, you’re moving in a plane just like the cursor would if it were moving. And anyway, the fact that this game is so mechanically similar to so many Flash-based games on the web, which generally don’t lock the cursor in place (because that would be really annoying on a web page), means that I’m aware, as I play, of the inversion from how these things usually work. That’s why I describe the mark in the center as a “cursor” rather than as a “reticle”. And the fact that you click on things to open up sub-menus (generally either “choose a type of tower to build here” or “upgrade/sell”) makes it seem even more cursor-like.

The 3D modeling isn’t just window dressing. The curving paths that the invading aliens come in on can cross over and under themselves, like in Zuma, with the result that you’re not just concerned with level geometry, but level topology as well. 2D games are relatively easy to think about, because we’re good at associating information with locations, thinking “This area is secure” or “If the enemy reaches that point it’s time to take desperate measures”. Paths that go underneath the main playfield confound this sort of thinking. Sometimes the paths form a confusing tangle that you need to simplify by blocking most of the pathways off. And that becomes an optimization puzzle: 1And probably an NP-complete one at that. which pathways do you block off to give the advancing enemies the longest route to your base, the most exposure to your guns?

Probably the most interesting thing is the matter of what happens when an alien survives the gauntlet you’ve set up and reaches your base. The normal thing for a tower defense is to do is for the monster to knock off a fixed number of hit points or civilians (sometimes a higher number for tougher monsters), and then either be absorbed (as in Immortal Defense) or teleport back to the entrance for another run-through (as in Gemcraft). In Defense Grid, your hit points manifest as “power cores” that the aliens are trying to steal. This means that the aliens aren’t just trying to reach your base. They have to actually carry the things offscreen. Sometimes the exit is in the same place as the entrance, so that they have to pass by all the same towers twice. Sometimes it’s at the end of a completely different path. Sometimes the level topology is mutable enough that you get to decide how much they have to backtrack. From the way I’ve just described it, you might think that making them double back most of the way is optimal, but that’s not the case. When you kill an alien before it makes it out, any cores it carried drop on the ground and start inching their way back to the base. While they’re on the way home, any other alien with carrying capacity to spare can pick them up. So a core dropped on the path in is just going to shorten some alien’s path. Furthermore, this means that the genre-typical swarms of individually weak creatures are among the game’s most fearsome adversaries toward the end of a level, because they make the panicked last-ditch attempt to rescue your last core futile. As long as even just one member of the swarm survives, it can pick up the core from where it falls and keep carrying it away.

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1. And probably an NP-complete one at that.

Immortal Defense: Story

The story of Immortal Defense is told through the monologues that introduce each level. At the beginning, these serve as mission briefings, but this function drops off over the course of the game. The story and the gameplay are pretty much separate, as in most games, but few games make a virtue of it the way ID does. If you’re pushing on through the levels regardless of what you’re told is happening in the world, well, the player character is doing the same. Like you, he’s isolated from the in-fiction consequences of his actions. We’re going to be pushing deep into spoiler territory here.

At the beginning of the game, the player character, Subject K, has his mind catapulted into the psychedelic cosmic realm of Pathspace to defeat an imminent alien invasion. The game has six chapters; the invasion by the Bavakh armada is defeated at the end of chapter 1. There’s still no known way to get your mind back into your body at that point, which leaves K isolated from humanity. There’s mention of years passing between levels, time that you’re not aware of. K’s daughter, unborn at the time that he started the mission, grows up and has a daughter of her own. And how do they relate to K? There’s talk of how you’re a hero, a legend. Your alien Pathspace mentor, Pul Wat Aa, is actually worshiped as a god by his people, and it’s not hard to see that down the road. But there’s one thing they never openly acknowledge: they also regard you as a weapon. A weapon that has to be cajoled and manipulated, but still, a highly effective weapon, and one that it would be a waste not to use. And everyone, on multiple sides, wants to use you: for a while, most of the mission briefings seem to be of the form “Why did you do X? Y is more important!” By chapter 5, the granddaughter is asking K to destroy incoming vessels that haven’t been identified yet, just in case they turn out to be hostile. They naturally turn out to be a peaceful scientific expedition by your allies. Even after you learn this, you keep on destroying further expeditions from the same source. The question is raised: why do you keep on doing this?

For the player, the answer is a combination of “because that’s how you advance the plot” and “because there’s nothing else to do”. For K, it’s a bit more complicated, but probably includes the latter. At the end of chapter 2, Aa betrays you and your planet is destroyed, leaving you as a defender with nothing to defend. This begins the revenge-obsessed phase of the game, a phase that lasts for a very long time and involves a number of rash and counterproductive acts on K’s part, as he refuses to let war die down. But what else is there for him to do?

The destruction of your planet also raises a mystery: your body was on that planet. Without it, how is it that you remain in Pathspace? K’s disembodied mind is referred to on multiple occasions as a “ghost”, and that starts to seem literal here. The mystery is in fact quickly solved: a number of your people, including the granddaughter, escaped the destruction, and eventually return to bring new life to the planet through nanotechnology — the same nanotechnology that they’re using to keep themselves alive indefinitely. This gives you something to defend once more, but at the same time, it seems too good, too perfectly wish-fulfilling for K, who regrets never getting a chance to meet his daughter in person. And indeed it all turns out to be a delusion. This is the reason that K destroyed those science fleets: they threatened to discover the truth. But even once this is undeniable, the hallucination of the granddaughter (whose name we’ve never learned) intriguingly argues that the delusions of an immortal are more enduring than mere flesh, and therefore more real. And it’s hard to argue with that from K’s perspective. Everything else around him is going to spend the bulk of eternity dead no matter what he does, including his fellow Pathspace defenders who are still dependent on their physical bodies.

It all reminds me a bit of the second volume of Tezuka’s Phoenix, in which, about halfway through the story, one of the characters is granted immortality. Suddenly the story takes a step back, and all the human conflicts that drove it up to that point fade in importance, as years pass, and millions of years. Something similar happens here, with thousands of years passing between levels, and the old factions and alliances disappearing and being replaced with new things that you’re no longer even given a chance to keep track of. The only thing that remains constant is K’s tenacious and pointless defense of his dead world. By the end, he’s descended into full-bore Jack Torrance insanity, to the point that I have to wonder if the final levels, in which all the boss monsters of the past return in large quantities, are supposed to be “real” at all, or just more hallucinations. (You have to wonder when one of the last ships types introduced is called the “:P”.)

Patrick Dugan wrote of the ending:

“I love you grandpa” is a piece of text that haunted me, leaving me shaken with wonder and existential horror, for hours after I finished the game.

And while I was skeptical on reading that, I have to agree: seen in context, as the last word going into the final mission, it’s devastating. But it isn’t really the last word: at the end of every chapter, there’s a bonus round in a simulation run by Jamesh, the inventor of Pathspace technology, and the final chapter is no exception. Here at the end, his words are a return to rationality, a frank discussion of what you’ve done and his own role in making it possible. And that final step back is the really masterful touch. The author of this game has thought about what it all means, and he wants you to think about it too.

In the official FAQ, the author states:

I put [K’s obsession with goals] into gameplay terms by making the last campaign of the story a direct challenge to the player: the missions are getting harder, K is becoming obviously crazier and crazier, and the player understands that there’s no point in world of the game to what he’s doing. The player can “win” in a perfectly acceptable way by just ceasing to play in those final moments: he can set the game aside, never pick it up again, and that means that K has come to his senses and abandoned his efforts.

I have to say this is wrong-headed. From a player’s perspective, abandoning a pre-scripted story in mid-game doesn’t change what happens in the gameworld any more than stopping reading a novel before the ending changes what happens in the world it describes. Even losing a mission, which in theory could allow the Bavakh invasion to succeed, doesn’t seem like something that happens in the “real” story of the game. There are games where the sense of what really happens is flexible, but this isn’t one of them. But as the same FAQ says, “I’m still on the fence about this–which is why you can also achieve a certain kind of victory by finishing.”

Apparently there’s a seventh chapter, set in “Hellspace”, that only becomes available if you complete every mission with a 100% survival rate. I imagine I’ll try for that eventually, but I’ll be surprised if it adds anything significant to the story. Pleasantly surprised, but surprised.

Immortal Defense

immortal-understandingI’ve known since I tried the limited demo of Immortal Defense that I wanted give the full game a try at some point, and with the author putting it on a temporary “pay-what-you-want” sale, now seemed like the time. (The sale seems to have a few more hours left as I write this, although the phrase “until January 1st” is kind of ambiguous.)

The premise is that hostile spaceships travel interstellar distances through “hyperspace”, but that a higher-order space called “pathspace” exists beyond this, in which hyperspace routes are visible as twisting lines. You send your disembodied consciousness into pathspace to place circular “points” (that is, towers) based on different aspects of your personality: Fear points temorarily stun their targets and eliminate their defense, Pride points increase in power as they kill more enemies, Love points don’t attack but increase the range and power of any other points nearby, etc. It’s more continuous than most tower defense games — the path isn’t based on a grid (except to the extent that everything on a computer screen is), and the points can be placed freely, as long as they don’t intersect with the path or each other.

It’s all very abstract. Not many of the spaceships look like spaceships; the most common ones look like organic globules encased in transparent bubbles or polyhedra. The points are glyphs, the weapon fire is a cascade of bright lights. Sometimes, especially in the more advanced levels, there’s so much clutter on the screen that it’s hard to see the cursor — and that’s important, because the cursor is actually one of your weapons. It constantly fires weakly at the nearest target, and can be used to direct the fire of certain of the Points. One of the perennial questions in tower defense games is “What does the player do while waiting to build up enough cash to buy a new tower or upgrade?”, and this is ID‘s answer. But when the cursor is difficult to see, game is played more in the setup phase.

In fact, that’s typical of the gameplay as a whole: any tactic you come to rely on is rendered less useful at some point. Take those Love points. There comes a point when you start relying on them heavily, clustering all of your points together so they can take maximum advantage of the bonuses. Then the enemy starts turning out ships that can disable nearby points. Suddenly putting all your points close to each other is a bad idea. When you get the final point type, the Danmaku point, it seems like the final ultimate invincible thing that will win the rest of the levels for you; at the stage I’m stuck at now, close to the end, I’m using it as a decoy.

Another unusual thing it does: it lets you carry over cash from one level to the next. Not always, mind you — every fifteenth level clears it — but usually. The result is, predictably, positive feedback: once you start doing well, you can afford to do keep on doing well with minimal effort, at least until you reach something that requires new tactics, such as a boss fight. It’s a classic back-and-forth dynamic: things get easy, things get hard.

But the most notable thing is the story. The author claims that it’s “the only tower defense game with a story”. This may or may not have been strictly true when the game was originally released; it certainly isn’t true today. But it’s certainly got the most interesting story I’ve seen in the genre, and one of the more interesting I’ve seen in games at all. I’ll get into that more in my next post.

Plants vs Zombies

I’m almost a week late with this post: I purchased Plants vs Zombies, Pop Cap’s foray into the Tower Defense genre, on Monday, and played all the way through Adventure Mode that day. (I’ve noted before now that recent Pop Cap games tend to treat the “adventure mode” or “story mode” as a kind of brief prelude to all the special challenges and minigames that occupy the bulk of the player’s time. Nonetheless, I regard adventure mode as enough to get it off the Stack.)

Tower Defense is one of the few genres born on the web, and still doesn’t have a great deal of representation outside of ad-supported Flash game sites. Perhaps this is why there’s still so much experimentation going on within the genre: there hasn’t been a major best-seller that everyone strives to imitate. PvZ‘s biggest experiment is splitting the playfield into several lanes that are mostly isolated from each other. (There are some effects that affect the lanes adjacent to where they’re used, but they tend to be expensive.) I didn’t care for this mechanic at first — it seemed like it just invited symmetric action, placing the same defenses along all lanes, which seemed like mere busywork. But it becomes more interesting when you’re short on resources, and still have to defend all the lanes separately and simultaneously. If you can’t afford what you need to destroy the zombies on all of them, you might switch to delaying tactics on some of them.

The title may sound like James-Ernest-style random wackiness, but when you think about it, it’s based on the attributes required for the tower defense genre to work: the attackers have to be slow and stupid (or at least undeterred by certain death), and the defenders have to be stationary. Which is not to deny that the particular choice of plants and zombies for these roles was made for their wacky value. The whole game is full of Pop Cap’s signature deadpan goofiness, and sometimes even surprises the player with humorous enemy behavior, just like I Was In the War. Pop Cap’s sense of humor is such an integral part of their more recent releases that I think it’s worth remembering that they didn’t always have one. Their first major success, Bejewelled, was about as funny as Tetris. I was reading an article recently about how Valve’s Orange Box shifted the gaming zeitgeist back towards humor, and it’s definitely had an influence on PvZ: after you win Adventure Mode, there’s a musical number that’s a clear attempt at being this year’s Still Alive. But Pop Cap’s sense of humor was developing well before the Orange Box: Insaniquarium (2004), like PvZ, used jokey character descriptions of the various entities in the game.

In fact, PvZ has quite a lot in common with Insaniquarium (which makes sense, because it was written by the same people). One of PvZ’s unlockable minigames, “Zombiquarium”, references it quite directly, and there’s a Tamagotchi-like “Zen Garden” mode that’s basically a plant-based version of Insaniquarium‘s “Virtual Tank” mode. Both games have an overall mechanic wherein the reward for completing a level is usually a new helper species with its own unique abilities. But most of all, they involve similar activity. Insaniquarium is all about caring for fish that secrete coins, which you click on to pick up before they disappear, then use to buy fish food, additional fish, and upgrades of various kinds, including weapons to ward off piscivorous aliens. (They could have called it Fish vs Aliens.) Similarly, in PvZ, the “sunlight” that you use to plant more plants is mainly emitted in coin-like chunks by things you’ve already planted (usually Sunflowers).

Now, most tower defense games have some mechanic whereby money — or mana, or whatever it is you use to buy more defenders — builds up over time, allowing you to keep adding more defenders as the battle rages on. Perhaps the most common thing is for every attacker you defeat to yield some cash; this is the approach taken by, for example, Desktop Tower Defense and Gemcraft. But PvZ is the only tower defense game I’ve played that expects you to click on the money to pick it up, rather than just deposit it in your bank automatically. It definitely changes the tenor of the gameplay, making it much more active. In a typical tower defense, you spend a lot of time just watching and waiting and planning your plans. Here, you’re constantly scanning for things to click on.