Interesting question, nobody's asked that before! Realistically, you won't be able to create a FPS like Doom with this. To be honest you'd have to be exceptional at Z80 assembler to achieve that on a Z80 running at 3.5 MHz. Then again, one or two enterprising individuals have managed to write 3D games using AGD plus there's an isometric game written with the tool and I wouldn't have thought either of those possible. Nothing remotely like Doom though. You have to get a bit creative with sprites and blocks to do anything vaguely 3D-ish