IWAD
From 3DGE Wiki
An IWAD - short for Internal WAD - is the main resource file for a Doom-engine game, containing all the game's original sounds, levels, and graphics.
REQUIRED
3DGE needs an IWAD file in order to play; without one the 3DGE engine has no game data to use.
3DGE needs an IWAD from either DOOM 1, DOOM II, FINAL DOOM, HACX, CHEST QUEST, or HERETIC to function. 3DGE also works with FreeDoom (Freedoom1.wad and Freedoom2.wad).
Alternatively, you can set your own IWAD with the -IWAD switch.