One thing I'll note from screenshots alone, something that I still find weird nobody does here: the lighting looks very flat and uninteresting. Most custom SRB2 maps for some reason seem to have lighting an afterthought, which comes off as especially weird when Doom, the game this was made off of, usually had a lot of good lighting details in its custom maps. Some mappers would go to detail, adding darker sectors to indicate shadow behind pillars or doors; others would simply make the lighting of an area diverse and interesting, etc. Hell, I bet ol' Romero had a guideline about that.
Part of the reason I'm not really impressed with the visuals of that 2.2.x port of that Sonic '06 wad.