Sentinel: Mars Defense is a visually rich, science-fiction-flavored, fixed-path tower-defense game, the less interesting--but also less expensive--predecessor to Sentinel 2: Earth Defense. You command the dropship Sentinel, tasked with defending a colony on Mars from seemingly endless waves of remorseless aliens.
Following a traditional fixed-path format, aliens advance up a single (sometimes forking) route, as you use your limited resources to marshal an array of upgradeable towers to stop them across four different maps, making strategic decisions on which towers to buy, where to place them on a grid, and whether to sell or upgrade units. The enemies are largely unremarkable which perhaps explains why the game doesn't remind you of their characteristics or even names in the midst of an attack. The visuals, however, are rich and realistic, with the capability to pinch to zoom in on the battlefield.
What separates Sentinel: Mars Defense from the rest of this crowded genre are its use of barriers and Drones. Each path has fixed structures--walls with health that advancing aliens can damage--which adds another strategic wrinkle to resource management and tower placement. The game's Drones are expensive robots (you can buy up to three) that will either mine resources or repair your barriers. You can also earn interest at the end of a wave for unused resources, which encourages you to spend as little as possible.
Lackluster enemies, over-long and sometimes tedious levels (you will definitely use the fast-forward button), and a lack of in-game information keep this game from being as good as it could be, despite four difficulty levels and both campaign and endurance modes to encourage repeat play. (A blinking "Defend Earth!" button centered on the intro screen, which leads to the Sentinel 2 purchase page, doesn't make the experience any better.) Sentinel: Mars Defense is a decent game on its merits, probably worth buying for tower-defense fans craving yet another title, but it demonstrates that it's getting harder and harder to stand out in this extremely robust genre.