This is the largest project I've worked on, it's a one-man effort regarding code and design, but some of the sound and artwork was contracted out, as was the music. The engine was 100% coded by me from scratch in C++, and the game uses directx9. It's a PC game, but a mac port will follow in time.

The game is top-down 2D using 2D-rendered-out sprites (in many layers) from 3D art assets for stuff like the mechs and the tanks and turrets.
A very complex system of sprite lists and render targets that shuffle back and forth along with shaders and some variable level-of-detail stuff is used to compose the final images, and the whole thing runs quite happily on older PC's even at 1920x1200 res at around 60FPS (it's actually locked to go no faster).

The biggest problem was getting everything to work fine with overlapping objects and shadows (the objects are basically rendered using painters algorithm) whilst at the same time allowing for all the offscreen compositing for stuff like night vision, lightmaps, the shadowmaps for the terrain and of course all the particle effects. The code is pretty complex as a result. it could be way simpler, but then it would run a 2 FPS .

The game is currently in beta, and you can pre-order and grab the beta from here:

http://www.gratuitoustankbattles.com

Click here to view the iotd