Classic Doom Add-ons
Here are some levels for classic Doom / Doom II that you can download. There are a zillion files for these games out there spanning over two decades of non-stop fan activity, so this page will be focused on my own add-ons and perhaps ones that are hard to find, highly recommended or requested to be hosted here. If you're looking for general collections of Doom WADs, try any of the following sites:
Doom II Levels by ETTiNGRiNDER
Double Reject (Two levels, 2019-11-16)
"Blood Forest" and "Top Secret Research Facility", a couple of ill-fated vanilla Doom II maps that I designed for community projects in 2018-2019 but that failed to make it into a megawad release. Aside from their similar origin stories, the maps are unconnected and can be tackled individually, but I placed the larger/harder map second if you want to attempt continuous play. These are unapologetically designed towards a "Doom as Doom was in the 90s" ideal and should work flawlessly under vanilla conditions to the best of my knowledge. If you're looking for hyper-tight slaughtermaps made for and by speedrunners, or high-concept "Doom as art" projects, keep looking, you won't find that here. If you want more oldschool-style Dooming, then maybe it's worth giving these a try.