uniTower

Play it for 5 minutes, play it for 500 - it'll be satisfying either way.

There's only one tower and enemy type, so your score boils down to: 1) how well you can utilise powerful environmental effects when placing towers; 2) how complex a maze you build for the enemies; and 3) luck of the draw - how perfect your level generation was.

Levels are randomly generated as per a seed (except for tutorials/player created levels), so the fun comes in trying to find and master the elusive "perfect level". You are encouraged to share or borrow seeds from your friends/community, but the thrill is still present even when hunting alone.