The Stack

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.

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.

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.