Interestingly enough the Portal games leverage similar level design given to the Source engine, not only for the obvious user controlled portals but also to sti

Interestingly enough the Portal games leverage similar level design given to the Source engine, not only for the obvious user controlled portals but also to stitch levels together seamlessly, including impossible architecture (the long bridge leading to the glados confrontation in the first Portal shows the room you're about to enter, but its actually too small and shaped differently to hold the room.