It's a 2D game, and it uses sdl and mono. I don't know why they use SDL, but that's written in C, so the engine itself might be SDL.
You know, there's this wonderful thing called google. A quick search returns gaming juggernauts like Second Life, The Sims 3, plus a metric ton of games written in XNA and Unity3d.for the pc platform which game has been written in C#? i really curious about this.
You know what the interesting thing is? Second Life turned to Mono in order to improve performance. Not only that, but they've used Mono to implement some positively awesome things like microthreads (Mono.Continuations) or the ability to record the current state of the engine, wire it to the server or another PC and continue execution there. There was a great discussion on the Mono mailing list recently (around last April) - check it out if you are interested.