I always loved mapping and ROTT is no exception. Together with Wolf3D, Blake Stone and others, it's really easy to map (so much that Wolf3D, Spear of Destiny, BS: Aliens of Gold and BS: Planet Strike share the same editor [Mapedit, not RottED, it's the same thing though]). It's as easy as messing with MSPaint.
I've got back into mapping, started making some levels and I have to say, it's fun. I'm currently using RottED which you can get at the 3D Realms' ROTT page (which has other editors if you want to try them out) or directly from here.
If you're a fan of ROTT, I recommend trying out the editor.
(Picture of a level section in the editor and its in-game counterpart)

Oh yeah, i love mapping for ROTT too! I used to make lots of levels for it, hmmm, probably now that ROTT has a brand new audience now, I should try to find those levels i made. Yeah, wolf3d map making is one of the easiest, along with ROTT, since its engine is based on wolf3d's.
Are you making levels for ROTT now?

Yeah, I am. Hell, I might even release it if it goes well.
That picture is from one of my maps. :V
For anyone that uses RottED, am I the only one that's getting a bug where the scrollbar disappears? It's impossible to map without the scrollbar.

This is cool! I remember making maps for Blood but never for ROTT, I gotta check this out. =D

I currently have 4 maps done. It's good if you open the original maps to get examples, I learned most thing that way.
I still haven't figured out how to add fog, I add the fog tile at X3,Y0 yet nothing happens.

To add fog you put the fog tile at X2, Y0, instead of X3, Y0. : )