Bought this, already played it a bit, then came here to recommend it. I'd have bought a 4 pack for copies for friends if it was possible. My last TD recommendations were Sol Survivor and Sanctum. This one is different (single player only) but very good and absolutely charming as hell. BUY IT.
http://store.steampowered.com/app/93200/
http://store.steampowered.com/app/93200/