The farther you progress into the game, the more powerful the foes you'll encounter become. You come face to face with man-eating lions in the coliseum. As you make your way into the catacombs, you will likely encounter minotaurs, leviathans, sapper demons, and guard devils. Later, as you descend into the bowels of Hell, you'll wage war against the hounds of Hell and fallen angels. The final conflict pits you against the prince of darkness, Satan himself.
Demons often attack in packs.
Devils provide a considerable challenge. At the outset of the game, you are visited by an angel of the Lord who bestows upon you the sword of the spirit. Don't expect to start hacking away at Roman centurions, though. In this game, the battle is fought on a spiritual plane. Tactics that work against inhabitants of the physical realm would have little effect on demons.