Currently 50% off from Steam and Amazon for $7.50
http://store.steampowered.com/app/65800/http://www.amazon.com/Trendy-Entertainm ... B005VFHOOEDifference between the two is Steam you get TF2 and a Portal gun for Dungeon defenders (engineer(heals towers), medic(heals players), heavy(increase player damage), and pyro(increase tower damage) pets. Portal gun for ranger) and Amazon gives you a pet that attacks, heals towers, and boosts your stats. Similar to the Engineer by the sound of it.
A little run down of what it is. It is a class based tower defense with a bit RPG mixed in. You level up, grab loot, upgrade loot, and work together with each class.

It's a really great game for $7.50