A mammoth resource back in the day for creators looking to make their own levels for DOOM, DOOM II, and (to a much lesser extent) Heretic. Although the information on the various WAD authoring and editing tools is quite outdated (there are much, much better programs available for modern systems that also work under the enormous variety of source ports), if you're looking to mess around with the internal workings of DOOM then there's still plenty of information about level creation that is valid