In his latest article on Tales of the Rampant Coyote, indie developer Jay Barnson discusses how dungeon design has evolved over the last 25+ years, from early tabletop modules to modern day 3D CRPGs.
The early first-person-perspective Computer RPGs worked well translating the old 2D graph-paper maps. These games offered four directional views in a tile-based world. They really couldn't show curves or any walls oriented in anything but the four cardinal directions, but they got the job done okay.
And of course, the top-down or isometric view CRPGs worked fine with these kinds of maps, although again everything worked best at nice 90 degree angles without very many exceptions. The descendants of these latter games still stick to the nice 2D, rectilinear maps pretty well, though they often offer more interesting wall and floor shapes and height changes.
But Ultima Underworld was the game that started screwing everything up.
For the most part, it stuck with safe 90-degree angles and so forth too, but it added the vertical element to its maps. And underground rivers. And occasionally broke out of the rectilinear world to give us 45-degree corridors. Our worlds and perspective would be forever changed.