you should get stuff for completing achievements. Also I think it's a very nice twist to normal tower defence games, and I especially like all the things that don't have to do with the game objective: killing sheep and goats, catching fish, freeing sasquatch etc.
 "Contract Wars"){kind=icon}
 "Mastermind: World Conquerer"){kind=icon}
 "Kingdom Rush"){kind=icon}
 "GunBlood"){kind=icon}