Tower defense games are finally starting to break out of their mold. We're now seeing interesting takes on the genre, where you control the attackers instead of the defenders, or even go one-on-one with a live opponent. But what could be the most interesting innovation is in adding a simultaneous action element to it all. Robot Entertainment's Orcs Must Die! isn't out yet, but if you're looking for something a little more sci-fi and, well, actually available right now, then Sanctum might be worth a look.
Sanctum is a first-person action game layered on top of the classic tower defense formula; it includes three levels and three weapons, along with quite a few different types of towers you can use and some unique (and altogether weird) enemies of all kinds to shoot down. Much like with Borderlands, you can get critical hits on monsters if you aim at their weak spots, which often lies in the vaguely face-like area on the front of the oncoming horde. Two-player cooperative play is supported and it's a blast to play with someone else in the same room (mostly because it's hard to communicate your brilliant design for creating the longest possible route for the monsters to travel), although I found that people rarely stayed in-game for long, quitting out and dumping me unceremoniously (and immediately) back to the main menu.
I quickly found that a losing strategy can still take you far if you've got good aim, but it can screw up your run at unlocking the next level (of which there are only three in total, so I'm not sure why there's even a need for this), and you might have to start over at wave 1 on a level if you don't want to waste a ton of money on refunds. And what I mean by that is in the money lost if you sell turrets back in trying to reshape or "fix" your setup. In the first level, the enemies will take a not-terribly-linear path to the exit, so it's up to you to block off a few narrow paths in order to force them to take the long way - and there's not a lot of space for creating the kind of zig-zag pattern that is so optimal in tower defense games. And sure, the cement chunks you use to block stuff off can be refunded for 100% of their value, but you take big losses on the turrets that are placed in or on top of them.
You do get checkpoints once in a whiel as you go through the many, many waves on the way to unlocking the next of the game's three levels, but there's no way to restart just a single wave if you know you didn't make it. In fact, be careful; hitting "Restart Level" will erase your checkpoint and start you back at wave 1.
You've got three main weapons in your arsenal, each with unlimited ammo - but they can and will overheat. There's a standard rapid-fire assault gun with a grenade-launching alternate fire, a sniper rifle with a zoom, and a grenade launcher that either slows down or freezes enemies entirely. It's up to you to develop a strategy to use your guns' cooldown times as best as possible, and to position yourself to hit critical zones on the monsters with the most powerful shots. To that end, you can always hit the Tab key and click on one of several pre-defined spawn points around each map.
When I look at Sanctum, I see an interesting art style and some fun action, but for the $15 price tag, there's a lot of untapped potential as well. It's not just in the lack of levels or overall content, but also in the types of enemies, the two-plane system (yep, there are flying enemies) stuck in a fully-3D world, and in the limitations brought out by your guns - even though they are upgradeable and can do tons of damage by the end. There's also something extremely frantic about this game and the constant perspective shifting you'll have to do just to survive later waves can be a bit disorienting; this is clearly not a game for casual players, but it's also missing deeper features for super-hardcore players. Sanctum isn't for everyone, but for a dedicated pair of FPS/strategy-loving people in the same room, it's a damn fine choice for for a fun weekend of innovative tower defense.