Immortal Defense

I've made the acquaintance of a new genre of 'mini-games' lately that I find rather interesting - 'Tower Defense'. You build towers along a set path along which your enemies are forced to advance; your towers can hit the enemies, but they don't usually have the ability to hit you back. They attempt to get through the path to the end, where they can assault your undefended base/civilians/treasure; you try to destroy them all before they can overwhelm you with speed or numbers and strength.

A good introduction to the genre is David Scott's Flash Element TD. At first the struggle is merely to survive; then when you notice other people's high scores, you'll wonder my gosh, how did they get so many points? and start thinking about how to improve your score, by minimizing the amount you spend on your towers while still being able to defeat the enemy.

( So I thought, hmm, a space-based tower defense game could be fun... Then I checked Google. )