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Sentinel was one of the better tower defense games on the App Store, so what does Origin8 do for an encore? They make the best tower defense game on the App Store, Sentinel 2: Earth Defense.
It seems that your best efforts in the first Sentinel were not enough, so now the Aliens have taken the fight to Earth. Sentinel is a fixed-path tower defense, so the goal is to defend your base from numerous waves of aliens as they traverse preset paths. Sentinel 2 has seven towers, and while some are genre standards, there are some creative towers like the ion tower, which creates a fire on the path that will burn oncoming aliens. Towers can be purchased using money earned from defeating aliens. Each tower has three upgrades, the last of which is disproportionately powerful and accordingly expensive. Each map contains intermediate gates and a final barrier, both of which are equipped with machine guns. Keeping your intermediate gates intact adds to your score, and if your final barrier is broken through, you lose. In addition, after each round, gates beam up energy to the Sentinel. Energy is used for utilizing one of the Sentinel's four weapons: homing missiles, attack drones, time warp, and orbital laser... all of which affect the whole map and are extremely powerful. Also available for purchase are two different drones: repair drones and energy drones. Repair drones will either repair gates or mine for cash, and energy drones will send energy to your ship. The enemies are varied and some contain immunities and resistance to certain towers. On a whole, the gameplay of Sentinel 2 is extremely deep and multi-layered.
Sentinel 2 contains two main game modes: campaign and missions. In campaign mode there are four maps with a set number of waves, and completing one map unlocks the next map and endless mode. Each map has four difficulty levels, the last of which, Psycho, is incredibly difficult. Missions are a new addition to Sentinel, and they are a welcome addition at that. Each of the ten missions throws you into tricky scenarios such as being very low on resources, and you'll have to use all of your skills to defeat the aliens. The game is also fully integrated with online leader boards via the OpenFeint service.
Placing towers is done using drag-and-drop, and the selection of towers is displayed at the bottom of the screen. Tapping on a tower allows you to either upgrade or sell it. You can use the standard Apple pinching gestures to zoom, however you cannot zoom out anymore from the default zoom, meaning that the list of towers at the bottom of the screen blocks a section of the playing field, so you'll have to zoom in and pan around to place and upgrade towers in that area, which can be very annoying in the heat of battle.
Graphics are among the best in the genre as well. The sci-fi theme is fantastic, and everything is very well detailed and looks good at all zooms. The animations are very smooth and well-done. Music contributes greatly to the sci-fi vibre, and sound effects are top-notch.
At $2.99, Sentinel 2 is simply a steal. The games strategy is incredibly deep, and there is enough content to keep anyone satisfied. Fans of the tower defense genre would be crazy to miss out on this one.