The sheer depth, numerous enemy types and levels, interesting systems in play, cute pixel art, juicy sound effects, cool achievements, progression, upgrades, traps, etc combine to make the most delightful tower-defense game I have ever played possibly better than Orcs Must Die!
Not sold on the ability button though, sure it might help in the early game but it just feels like too much busy work and counter-intuitive as to what makes a tower-defense fun.