OTTTD stands for Over The Top Tower Defence, and you'll see why when you're blasting 9 kinds of hell out of a Cactus Ghost that's making a beeline for your battlefield HQ. You play as a commander for HEROCORP, the 4th largest private military organization in the galaxy. A long period of peace has left the company coffers a little bare so the science division developed Trans-Dimensional Rifts, opening up lucrative opportunities for war in other universes. It's like Starship Troopers meets Stargate meets Spaceballs, with way more explosions. 
