DooM is not only fun, but it's great fun to play around with from the inside, or even to port. I've helped bring Doom to the x68000, which was quite the mission as the only compiler we had that could build 'large' executables was GCC 1.39 which had been translated into Japanese.

But it's been fun doing ports to OS/2, DJGPP v1, Watcom C, and of course various other Unix via the X11 code. The GPL'd code was 'cleaned up' and kind of bugged up on the way, sadly what was released really wasnt' all that pure.

For anyone wanting something more 'pure' for MS-DOS I'd highly suggest: Mara'akate's DooM-New

https://bitbucket.org/maraakate/doomnew-for-dos

And then there is Quake, which I compiled with the excellent MS-DOS based TCP/IP stack WATTCP so that I could bring native TCP/IP networking to Quake.

https://virtuallyfun.com/wordpress/2011/01/05/quake1-with-wa...

And on the heels of that, I did a QuakeWorld client for MS-DOS

https://virtuallyfun.com/wordpress/2011/01/06/quakeworld-cli...

which kind of took a life on it's own here:

http://dk.toastednet.org/QDOS/index.html

And of course with the source to Quake 2 available, it only seemed proper to port it to MS-DOS.

https://virtuallyfun.com/wordpress/2015/06/02/porting-quake-...

https://virtuallyfun.com/2015/06/03/porting-quake-ii-to-ms-d...

https://virtuallyfun.com/2015/06/07/porting-quake-ii-to-ms-d...

https://virtuallyfun.com/2015/06/26/porting-quake-ii-to-ms-d...

https://virtuallyfun.com/2015/09/17/porting-quake-ii-to-ms-d...

So yes, there is GREAT fun to be had in the iD source.

Also don't forget some of their earlier games:

https://github.com/CatacombGames

https://github.com/FlatRockSoft/