Like holy shit how. The Doom engine's normal level building doesn't take into account the Z access, and sure you can remove limits, and do crafty tricks to get around not having sectors over sectors in REALLY cunning ways (doom 64 did this a few times to GREAT effect!, but there are EVEN better ways it turns out, with some technical wizardry - and access to the source code >w> )... but fuckin'... curtains/banners blowing in the wind? grass? slopes and curves and all that shit? holy fuckaroons.