Descent. Anybody else remember the glitch that allowed you to create pseudo-4D maps?
The maps were make up of blocks, the edges of which you could stretch and skew however you like, as long as you make sure the edges of adjacent blocks join up with each other. The trick was that you could exploit the joints of the blocks to create overlapping blocks within blocks that appear differently in game depending on your point-of-view.
Once you got the knack of exploiting this, you could, for example, make a small 8-sided cube suspended in mid-air covered with doors, and each door would open to entirely different rooms. Awesome stuff.