You have to infiltrate Simons' base and fight against his personal security force that is made up of cyborgs, genetically mutated humans, combat androids and even more types of enemies. You can rely on an awesome arsenal of weapons to crush your enemies.
The gameplay is very close to Gears of War's, since you can advance through closed environments while you can hide behind any cover possible and shoot at the waves of enemies that come at you.
The quality and style of the graphics also are quite similar to Gears of War, since they're both pretty great for an iOS game and the setting is also very similar.
Shadowgun is an outstanding game with great production value for a cell phone's desktop console and a great gameplay that will keep you hooked for hours on end.