Revenge of the Titans is a tower defense game in which enemies come from all sides. Players determine which buildings are available through an extensive tech tree, and can place them anywhere on the level map during gameplay. There are four play modes: Campaign, Survival, Endless, and Sandbox.
The goal of each level is to destroy all the alien attackers while preserving the integrity of the base structure. Apart from the base, the most basic building is the Refinery, which can extract resources from specific parts of the map; these resources are used to place more buildings, including auto-firing gun turrets and click-to-shoot lasers. There are a broad variety of structures, some of which solely enhance the functionality of others, and there is a correspondingly complex strategy involved in passing each level. I'm impartial to the droid and tank factories, but much as I like to have swarms of little robots blasting away at aliens, relying on just one type of combat structure will nearly always result in defeat.
Sam's Related Factoid: The game's title references Titan Attacks!, a Space Invaders-style game by the same developer. Revenge of the Titans could be considered a sequel, even though the gameplay is completely different.