Originally Posted by Cammy
Desert level (Sometimes also a Pyramid level)
Slippery Ice/Snow level
Perhaps the reason for this is simply to supply the player with different settings, and in turn different things to look at.
Having to play through a game set in the same environment becomes monotonous quite quickly - look at the original Wolfenstein, for example. And then see how Id tried to spice things up graphically in its successor, Doom.
As for me personally, I'm trying to write a fantasy novel at the moment, and I've slipped in a few different environments, purely for the reason to keep things interesting for the reader.