all of the monsters and weapons that are from the doom dos games are replaced with the rpg versions.
some monsters are locked to a certain difficulty. there are 3 difficultys have easy to hard versions of the monsters.
the shotgun guys are replaced by an upgraded zombieman, as there are no shotgun guys in the rpg game. so getting ammo for the shotguns might be difficult.
If you have any questions or helpful tips. place them in the comment sections.