You can find the source in their git here.
Catacomb,Â The Catacomb,Â Catacomb 3D,Â Catacomb Abyss, andÂ Hovertank3D
However this does not include the data files (of course). Â Also this is not 100% of the source code, as I noticed that libraries are required.
I haven’t taken too much of a deep look though.