Compiled by iD Software in response to various commercial DOOM wad collections like D!Zone, the Master Levels for DOOM II was intended to be the largest mega-collection of additional Doom maps (the “Maximum Doom” collection), plus twenty new official maps (the “Master Levels”). The “Master Levels” are twenty stand-alone maps from independent level designers commissioned by iD. There is no overall campaign or an attempt to tie the levels together, though some of the levels from the same designer do have a connected story of their own. “Maximum Doom” is a collection of 201 .wads (map packs with varying numbers of levels inside) for the original Doom and 1,629 wads for DOOM II, leading to the package’s claim of 3,000 total new levels. These wads are not commissioned by iD or made by professional designers, and no claim is made as to these being the “most popular” or “best designed” homebrew levels available. The maps were gathered from various free or FTP sites on the Internet; however, the time and dial-up cost for a player to find and download the same collection of .wads themselves would have been substantial.
description from MobyGames