The Build Engine was created by Ken Silverman in the early to mid-1990s. The Build Engine was a huge step-up from the Doom Engine by providing many new features that were never seen before, such as destructible environments, sloped floors/ceilings, mirrors, looking up/down, flying/crouching/underwater, voxels and 3D immersion. However, the Build Engine is not a completely 3D game engine, since it utilized a 2D grid for world editing and added a height component for its sectors. This height component allowed each sector to have a different ceiling height and floor height! Floors and ceilings can hinge along one of the sector's walls to create slopes! Tomorrow I'll talk more about Build's Sectors and Portal System. I just wanted to quickly introduce this topic today since I've been busy setting up for school, hence the short posts recently!
Here is a video that overviews what the engine is capable of!
NOTE: The quality is low on this video though.