In a tower defence game you face waves of enemies, slowly increasing in difficulty while you build up your defences against each wave. There are usually waves with different characteristics (i.e. some can fly so are immune to ground based weapons) and weapons to match lots of characteristics. The idea is to build up a mixture of defences to defeat al enemies.
The premise here is that you are defending the Mars base from invading alien hordes.
One of the things which hits you immediately about Sentinel is the high quality visuals and sound effects. Of all of the games in this genre which I’ve played this is by far the best produced. It’s also one of the most addictive. There are 4 separate layouts to defend, in either assault (90 waves of attack) or endurance (unlimited waves.)
Since the release of Sentinel 2 (review coming soon), Sentinel has been reduced to 99c – well worth the money.
My only bugbear with the game is that it stops whatever you’re listening to when it starts but you can get around that using the double-click trick.