It's based on the Doom 3 engine, which they customized further (a lot).
ID released the source code of Doom and they've been working on this for years - I've been playing it since 1.0 I think.
Now it comes in x64 flavour, too.
Runs on Linux, too if that's your poison.
I guess you should be able to get the source code of TDM, too - seems like you already did? (I never tried though)

There are some pretty decent fan missions out there, which makes it great.
Just download the Updater EXE file, drop it in any directory and let it access the internet - it will download the game.
Within the game you can download the fan missions.

Here's a table I made for 2.07, might still work for 2.08.
Just a few scripts, similar ones as before, except for hearing, that's missing.
Table includes some debug stuff, too (structures and code points), those could help updating it, if need be.

Author: Csimbi

• Thief: Table for Cheat Engine {Csimbi} [2021-Aug-19]