GeneticInvasion is a tower defence game under a free license.
What makes it different from all the tower defences you've played so far is that is uses genetic algorithm to make the enemies evolve.
No more scripted increasing difficulty system.
The enemies adapt themselves.
You improve your defence, they are going stronger.
You have a minefield to protect you, they begin to fly.
You build a whole lot of anti-air turret, they stop flying.
You can guess how that ends : you lose.
GeneticInvasion is still in alpha stage, so do not expect it to be perfect, bugless or even crashless.
I suggest you to play the levels that begins by a number, they are supposed to be balanced and are sorted by difficulty (bigger number = more difficult)
The game already has a medal system :
No medals means you suck
Bronze medal means you figured out how to play
Silver means you're good
Gold means you're really good
Platinium means you're too good, play a harder level!