Sentinel

I started playing Sentinel with jaded expectations, I'm afraid. Another tower defense game? Don't get me wrong; I love the tower defense genre. But I didn't expect Sentinel to be exceptional, and I was afraid of an experience like, oh, Circuit Defenders.

Thankfully, Sentinel is much more than just another clone. The graphics are possibly the best I've seen in a game of this genre, and there's enough uniqueness to keep genre veterans and newcomers alike hooked.

The Story
Sentinel is set on Mars, and the story is presented in a familiar scrolling bit of text when you start the game (this intro can be skipped, though). Humans have established a mining colony on Mars, an invasion happened, you (the captain of the dropship Sentinel) were sent in to fix things up, and so on...and now there are hordes of aliens that require some good old-fashioned bashing. Oh, dear.

Okay, so the story isn't anything special or involved, per se. But it does provide justification for the game's existence and for its graphical presentation, which is all that really matters.

Gameplay
For those unfamiliar with tower defense games, they basically work like this: enemies pour from one point and flow down a set path towards your base, which is at the end of the path. You set weapons (towers) along the path, and they fire at passing enemies. Let the enemies destroy your base, and you die.

Sentinel mixes a few things up by introducing different structures and some other quirks. You have a number of structures (a base and one or more barriers), but they each have a health bar that will be depleted as the aliens hack away. So, instead of placing towers willy-nilly, you'll have to decide if you want to concentrate on keeping your barriers alive or if you want to scatter your weapons throughout the level. The maps also allow for multiple enemy entrance points, introducing another element of strategy: should I focus on the separate stream of aliens, or should I wait until they merge? Also, the fact that both bases and barriers have health bars adds some more strategy and tension—enemies keep attacking your structures until you kill them, instead of just doing X amount of damage and disappearing, which means that you have to kill them off quickly.

Sentinel comes with six different towers, though in this game they're called turrets. To create them, you simply drag their icons onto the map and set them down. There's also a seventh machine--a drone, which will repair your structures or harvest resources for cash. Each turret can be upgraded up to three times (for a price!) or sold by tapping on it, and each has its own strengths. Some maps limit your arsenal to a few different turret types, and diversification is necessary anyway (the bombs, for example, don't hurt air-bound aliens). The turret variety is good, though not exceptional. Also, the cash that you don't spend on turrets can earn interest. It's a very small amount of interest, but it does make you more inclined to save rather than splurge.

As far as the enemies go, you have your basic types: ground, flying, fast, slow...nothing too surprising. There are "bosses" in each map, too, and they're are particularly nasty to deal with.

There a few perks that are much appreciated. The pause button and fast-forward buttons are both must-haves, but the ability to save your game and then quit is one that's often overlooked, and I'm glad to see it here.

Presentation
The graphics in Sentinel are nothing less than stellar. The animations are gorgeous and realistic, far more so than I've seen in other tower defense game. The framerates are perfect, the textures are good, and there's a great grunge theme that doesn't feel too overdone. Even the menu looks great. Weapon effects are explosive and colorful. You can even pinch and zoom. If you want eye candy, grab this one.

The audio is fine, too. The sound effects are fitting, and the in-game music matches the sci-fi theme quite nicely.

Things That Need Fixing
There are a few things that I think could use some work. For one, it's sometimes hard to select turrets in the upper-right section of the screen, as the buttons sometimes get in the way, especially when you pause and the other option buttons pop up. The number of maps available isn't incredibly extensive, either, and the folks at Origin8 ought to add some additional ones in the next update.

Also, the help files are somewhat thin. For example, it took me a little while to figure out that the droids are controlled collectively—at first I assumed that each droid could do one thing only, and that you decided on resource-harvesting or structure-repair when you bought the droid. Not so; you control all of them with the icon in the bottom-right. (I would like to be able to send each droid on a different task, but eh.) Also, the enemy types flash on the main "monitor" from the menu, but there's no enemy database or a quick reference guide.

Conclusion
In all honesty, though, those are fairly small concerns. Sentinel makes up for its minor (very, very minor!) flaws by pulling off a polished presentation that rivals every other tower defense game in the App Store. The graphics are stellar, the gameplay is fun, and despite the fact that I've got other TD games on my iPod, I'll probably be coming back to this one. Get it while it's priced at a mere two dollars!