Some of these tricks were used in the game itself (3D bridges and the sloped column trick as examples). Using sprites and slopes, you can fake true 3-dimensional objects.
3D Objects Using Sprites
Using wall-aligned and floor-aligned sprites, you can create 3-dimensional objects of almost any variety: switches, bridges, tables, chairs, even entire buildings! The only downside is that you can't slope sprites, and just about everything has to be square or rectangular.
Here's another trick to make sprites look 3-dimensional. The
only map I've ever seen it used in is BobSP3 by Bob Averill. I think
it's safe to say it was his idea. First choose the sprite which you
want to make 3D (place it in wall-alignment mode and press Tab
to copy it). Now (in 2D mode) place multiple wall-aligned copies on one
grid point, but at slightly different angles. In this example, I used
OCEANSPRITE5
(#975).
This makes the sprite look much more 3-dimensional. However, this only looks decent when used with certain textures.
3D Objects Using Slopes
Start by creating this setup. The two adjoined sectors should
each use the middle line as their 'first wall' (assigned with Alt + F
, shown as blue in the image). Block all of the walls surrounding the pillar, or else it could potentially kill any player who walks too close.
Raise the floor of the leftmost sector until it touches the
ceiling and slope it until the slope touches the surrounding floor. Do
the opposite to the rightmost sector: Lower the ceiling to the floor
and slope it until it's even with the surrounding ceiling. Texture it
and make sure everything is even. You may want to use E
on the sloped floor and ceiling to shrink the textures.