As I've mentioned earlier, the first few maps I made for the 'Acid Wars' project didn't have any of those fancy effects that GZDoom offers - they started out as plain vanilla maps. Later I converted those maps to Ultimate Doom Map Format, and started adding new features such as rotated textures, slopes and deep water.
The current map I'm working on was started using UDMF right away, and it's the first of my maps that features 3D bridges, even 3D slopes. This video tutorial helped me a lot.
Here are some screenshots of the results, first one is a simple 3D bridge, actually several 3D bridges:
A teleporter on a 3D bridge:
This room was a bit tricky to do, since it's a sloped 3D bridge with some 45 degree angles. The tutorial didn't cover odd angles, but after some experimenting I found out how to do that. Actually, the whole room was an experiment but it's quite cool so I'll keep it in the map. You can see another new feature in these screenshots, acid dripping from the ceiling (click the thumbnail for a fullscreen image, otherwise the drops are very hard to see).
More acid drops, and a new texture I made from a vanilla rock texture, it's basically the same rock but with acid flowing through the cracks. Looks quite cool when animated. It won't hurt the player like the nukage/acid liquid does.
Also, you can see another 3D bridge here.
No monsters in these screenshots, this map is very much WIP. The lighting looks rather bland, but it's a cave map for most parts so it's kinda hard to give it different lighting besides the GL light sources. Maybe I'll open some cracks in the ceiling for a better variety in light levels.