What platform? Search says Atari ST and then ported to the Amiga, Apple IIGS, DOS, SNES, TurboGrafx-CD, Sharp X68000, PC-9801 and FM Towns.
The PC version appears to have an editor alreadyhttp://dmweb.free.fr/?q=node/695
It says the others might come later (though that was 2005 and may be the latest release). Games of this vintage can share formats but they can also be quite different as things were done to play to the limitations of the machines. Still the triggers list at the end is probably worth looking at as you will likely have many of the same things (if not all of them) in whatever version you are looking at.
People even then used it to remake the game in things like grimrockhttps://www.nexusmods.com/grimrock/mods/65/?
I could cover level editing in general but I am not sure how useful that would be in this instance (no point in covering the layered approaches used by side scrolling games and I would want to cover that to begin).