*Walking through the portal at 10:45 is the last secret, though due to an issue with the level's design, it's not possible to get credit for triggering this secret without cheats.
Whatever. I got it. It counts.
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Also, you'll need a legit copy of your DOOM2.WAD file, either from a copy of your own discs, or from a digital distributor (I purchased & downloaded my copy of Doom II through GoG).
Add D44D, GZDoom, any other resource packs you've downloaded & your WAD to the same directory, double click D4D DragAndDrop to start the launcher, and you should be good to go! Have a blast!