Bobby was also fairly instrumental in doing this, and he even made an original level or two. Whenever a new idea would crop up, both of us would convert a few already existing levels to the new gamemode in order to see how it would work. One level in particular, which I think was called "Evil Castle" or something along those lines (it used the hocus pocus tileset), must have had at least 10 different versions. Also, for a lot of times we weren't even coming up with the gamemodes: they were being adapted from other games, or based on other people who had good ideas but lacked the ability to execute them (this caused some drama even though we never took credit for anything). That would have been 2001 or 2002. Later on BlurredD would basically be doing the same thing, but he largely stuck with making original levels.

I should also note that in the majority of cases, it usually stayed a mess. For every good gamemode concept there were two bad ones. Some of those bad ones got made into prototypes (usually small gimmick levels), but most were just discarded. This applies even more to levels themselves. If I don't like a level, I won't host it or release it, even if it's basically finished. I've never bothered to count how many there are, but there are a lot. More MP than SP now, since I recycled most of the discarded SP levels (or sometimes just remade them from scratch, saving the parts that worked) and wound up releasing them in my various episodes. Ep5 has bits that literally date from 1999.

The whole thing was really just a big process of trial and error.

I think this concept predates the level you are talking about. All I remember doing really was add falling TNT as a sort of flashy gimmick. The whole level took about 10 minutes to make. Black Ninja probably deserves more credit; I remember him hosting that level (or an edit of it) a lot.