These are ports of Keen Dreams, Catacomb 3-D (The Descent) and The Catacomb
Adventure Series. Although it may initially seem non-obvious, the reason these
ports are available under a single project is that a lot of common code can be
found. It may be true that Keen Dreams is a smooth-scrolling 2D platformer
game, while the Catacombs are 3D first person shooters, but there are still
common low-level 2D picture and font drawing routines, as well as very similar
user input and sound output routines. Files with such code are often marked
"ID Engine". A later revision of the codebase is also found in Wolfenstein 3D.

These source ports aim to reproduce behaviors of original executables
for DOS, including bugs, at least if it's not too difficult
(e.g., anything that highly depends on the memory layout).
With the exception of The Catacomb Armageddon/Apocalypse, this also includes
compatibility with saved games for original DOS executables (done per version).
The Chocolate Doom source port can be considered an inspiration for that.

This is a game engine - the data files are not included.