We are currently building a tower defense for Unity 3D in 2D! We are using Sprite Manager 1 to do all the 2d graphics. What you see in the image above is the stress test I just ran in the engine. Hundreds of turrets shooting at 10 damage a hit. Tanks spawning with 4000 hp at a rate of 0.03 seconds per spawn. The break you see is the 10 second wave break. 

I will be using this TD engine in the future to create a number of games. First game should be coming out within the next few months.