Competition is as stiff as ever to get noticed on the App Store. Nowhere is this more true than among the sea of tower defense variants available. Many of these titles have added a novel take to the basic formula, or gussied them up with flashy animation. Very few titles have managed to deliver across the board, but Sentinel 2 is one of those few.
In Sentinel 2: Earth Defense, you must defend Earth against an alien invasion. In addition to a fairly typical array of seven towers, you have at your disposal a powerful starship orbiting overhead and the ability to manufacture worker drones. These additions deepen the game and makes the experience feel closer to a real-time strategy game.
War. War never changes.
Your ship provides four different types of overhead attacks, which are activated by spending energy that you have accrued. These powerful attacks can get you out of a jam or buy you time to rearrange your defenses. You can also spend money to build drones to harvest energy and money or repair damaged defenses. Deciding when to spend your energy or invest in drones adds a lot of strategic options to the game and opens up multiple paths to victory.
There are four maps in the game. Each consists of a path that winds from an entry point to your base. The path that the enemies take might branch or be blocked by friendly barriers that can soak up a certain amount of damage. Since there is limited real estate on which to build your defenses, the strategy lies in how you choose to invest in your defenses, and where to build choke points to exploit the terrain of the level.
Each map has four difficulty levels and two game modes. These maps are also tweaked to offer a set of 10 special challenges with unique starting and victory conditions. This bevy of content is all linked to an online profile via the OpenFeint online network. As a result, the game comes with unlockable achievements as well as all of the online features that come along with OpenFeint, like online leaderboards, chat rooms and friends lists. Chasing achievements or going for a high score adds some replay value.
Alien barbarians at the gate.
All of these features did not distract from making the game look and sound great as well. The terrain, towers and enemies are all crisp and clear, and animate very well. You have the option of zooming the screen in or keeping the entire map on screen at once, which usually works fine. You can also speed up the game rather than watching wave after wave crash against your mighty defenses, and we feel this is a feature that every tower defense game should have.
Sentinel 2 also allows you to choose tracks from you music collection to make a playlist in game. Unfortunately, this feature does not seem to work on 3GS handsets. We could go on about the rich feature set in Sentinel 2, but suffice to say nothing feels left out.
The elegant interface manages to put all of these these features at your fingertips without feeling too complicated. Anybody that has played a Tower Defense game should feel right at home with Sentinel 2. However, it also has a pretty decent tutorial system to get uninitiated players up to speed. It is probably not quite unique enough to buy if you are already burnt out on the genre. Despite its polish and few twists, its core gameplay still feels quite familiar.
All in all, Sentinel 2: Earth Defense is an impressive package. It may not try to reinvent the wheel but it is a close-to-perfect implementation of the well-worn tower defense formula. It is well worth the $2.99 price tag and we would recommend it to anybody looking for a great strategy game.