The most important thing to note about Sanctum is that the first-person shooter/tower-defense combo really works.
There isn't much to the story: You play the role of Skye, a futuristic female soldier with a large cybernetic left arm who is sent to protect her home of Elysion One from what seems to be an endless supply of alien-ish enemies. But story in this game doesn't matter, as it's just a way to explain why you want to prevent said enemies from advancing from point A to point B, the "core."
The task is accomplished through typical tower defense tactics. The object is generally to guide the incoming waves of enemies through a maze of defensive structures with the intention of giving them adequate time to defeat the enemies before they reach the core at the end of the level. Gatling, Lightning, and Mortar towers defend against numerous ground enemies. Anti-Air towers are effective against aerial enemies, and the Scatter Laser defends against both. Though, not all towers are available on every level.
To oppose the variety of towers, there is a variety of enemies that come in numbered waves in an effort to reach the core. They each have their strengths and weaknesses, but they all share the same mostly-black-with-some-orange appearance. Some are big, slow, and powerful, and some are small, fast, and weak. Most are ground walkers while some are aerial. As with any tower defense game, the key is to obtain the proper balance of tower varieties in order to defend against all enemy types.
Sanctum offers three unique levels, each with its own landscape to populate with towers, with its own enemy entry points, and with its own total number of waves. The first map serves as a good introductory one, with some smaller corridors offering minimal flexibility so you can concentrate more on mastering the mechanics of the game. The second map is quite different, presenting you with a large, empty landscape, providing a significantly larger canvas to work with. The third level is a bit of a combination of the two, offering several smaller areas that culminate in a larger, open area at the level's end.
Of course, the big twist in the case of Sanctum is that the entire process is executed from the first-person perspective. Between each wave of enemies is an intermission that serves as the building phase. During this phase, areas where you can build a tower are marked with a blue square on the ground. First, you build a block. Then you convert the block into the defensive tower of your choice. Every built object costs points which are earned by surviving waves. You can also spend additional points on existing towers to upgrade them. With each passing wave, the amount of earned points is higher, enabling you to accomplish more between waves as the game continues on.
While fresh, the first-person perspective from the building phase can also be rather disorienting at times. As anyone who has played a tower defense game knows, it's useful to work from a tactical top-down perspective. Sanctum does offer such a perspective, as a push of the Tab button will give an aerial view of the level which goes a long way to help plan your structure layouts. However, it is a viewpoint only, and no actions can be carried out from it. While this does keep an emphasis on the first-person view, it's less excusable that this view is not even descriptive. It would be much more helpful to offer mouse-over tooltips that would at least specify the types of towers that are built and their levels. It does, however, allow you to teleport your character to any present "Televator" blocks, which makes traveling to different areas of the map painless.
But it's not just about doing the same old tower defense routine from a first-person perspective. Your character is also equipped with three different weapons: a rapid-firing assault rifle, a sniper rifle to damage distant enemies, and a freeze gun which slows them down. Just as you can spend points on building and upgrading defensive towers, you also have the option to spend points on upgrading your character's weapons.
Then, once you end the building phase and the wave of enemies begins, you take an active role in assisting your towers by firing on the enemies yourself with your weapon of choice. The resulting tactical options make this game much deeper than your average tower defense game. You can ignore your personal weapons and just build up towers, doing minimal damage to enemies yourself and counting on your towers to pick up the slack. But if you think you can take matters into your own hands, you can spend a little less points on the towers and instead crank up your weapons. This ups the replay value in a big way.
And you'll certainly be playing multiple times because this game is pretty hard. It's easy to get cocky because the first couple of waves will likely fly by without a hitch. But you'll inevitably be faced with a type of enemy that you hadn't been appropriately preparing for, and a lot may get through to the core. Situations like this can put you in a hole that it's difficult to get out of, as there is no way to "heal" your core through the course of the game. If things get too hairy, the game also includes a co-op multiplayer mode, and a notice when launching the game says that four-player co-op is planned for the near future as well.
It's easy to forget that Sanctum is an indie title. It's not just the gameplay, but the presentation in general is particularly well-executed. Running on the Unreal Engine, Sanctum's visuals are pleasingly bright and colorful. The background landscapes really give off the notion that you are on a lush, far-off planet, with nice touches like birds in the distance to fend off any sense of stoicism. The enemies themselves are a bit drab, but their dark colors contrast well against the vibrant backdrop. The ambient-ish electronic soundtrack does what it should in a game like this; it fills in the cracks without being distracting. And while the sound design might not be anything to write home about, it definitely isn't bad, and manages not to be annoying when there are a million things going on at once.
All told, Sanctum succeeds in proving that the FPSTD mechanic works, even if it also proves that said mechanic is in its infancy. It's still a tower defense game above all else, so this is more for fans of that genre than FPS gamers. And you don't have to be great at shooters to get a kick out of Sanctum. At only $14.99 on Steam, this indie title is a diamond in the rough. It's not perfect, as more options on the top-down viewpoint could have done away with many frustrations. It could also surely use more than three levels to retain its replay value. Regardless, there aren't many new titles available for the price that will offer as much satisfying gameplay as Sanctum.