Build styled cardboard sprites options. Sprites in Doom face the camera at all times, but cardboard sprites only face the direction they're pointing when placed in the map. There was also a cardboard sprite option to flip sprites down horizontaly, like a peice of paper, in build. These two tricks were used in many Build games along with K2 style bridges to give the illusion of 3D on bridges.
Example:
http://www.macgamefiles.com/imgs_screen/18448.jpg <-- Cardboard sprites
I couldn't find a good example of one of the bridges, but trust me... in game, you can't tell it's not 3D.