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I know I've reviewed a lot of tower defense games lately, but honestly, it's worth sticking around for this one. Sentinel 3: Homeworld by Origin8 is a masterpiece with great art and a lot of depth. This is the first game iOS developer Origin8 has brought to Android devices, and sadly the first two Sentinel games are absent. This one is widely regarded as the best entry, however, and it's not like there's much story to worry about.
The only story there is lies in the basic premise. After defending Earth and Mars in the previous games, you now take the fight to the enemy on their, wait for it… Home World! It's a classic set up. The game has you defending established bases from waves and waves of mutant and insectoid aliens. The way this game really sets itself apart is through the Commander. You still place towers along closed circuit paths to strategically destroy enemies, but you also have control over the Commander unit in his giant mech suit of doom. The commander helps fight off enemies through ranged and melee attacks, buffs any towers within range, and has several other abilities. Completing levels awards you with credits and experience. Credits buy new tower and weapon slots, while experience levels the Commander. As he levels, he'll gain new abilities, and you'll be able to increase his stats, such as health and damage. To add even more depth, you have orbital weapon strikes. These take up energy which is built up over time, and are great for getting out of messy situations.
Most levels have several barriers along the path to your main base. The Commander can jump between these barriers when needed, buffing new units and lending a helping hand. Each barrier has limited defenses of their own, but they'd crumble without help. There are also different types of barriers, such as the factory which brings in extra money each wave, or the energy tower which brings extra energy. Outside of the 20 campaign levels, you have Endless and Classic Mode. Endless Mode is exactly how it sounds, and you'll constantly unlock new endless maps. Classic Mode gives you a specific set of weapons and towers, challenging you to think outside the box to complete challenges. The Commander and all of his stats and abilities carry over between modes, and you can build up credits and experience in Endless Mode if things get too difficult.
The art is excellent, with an HD upgrade available to anyone with a phone that can handle it. The electronic soundtrack is also excellent, with some tracks that I think might even qualify as dub-step, maybe? OpenFeint achievements and leaderboards are supported, and the game can be yours for three dollars. This is one of the best mobile tower defenses I've ever played. It's much more traditional than say, the strange and unique Jelly Defense, but it brings a lot to the table, and I highly recommend giving it a go.