Erwin's Fortress begins as Pac-Man enters the tunnel into the fortress, right after where he left off in Cragstone Bridge. Pac-Man must navigate through large armies of Erwin's minions as they patiently wait to attack Pac-Man around every corner. Hidden throughout the level are puzzles that Erwin hopes will keep Pac-Man lost forever in his fortress. In addition, Erwin talks in a condescending tone to Pac-Man, Orson, and the purple creature (whose voice he recognizes) to intimidate them further. After much battle in Erwin's fortress, Pac-Man rides a lift to the adjacent city of Dungen Gunden on the inside of the fortress, which he explores in the next level (and which contains the "final" syphon).
As the longest level in the game, Erwin's Fortress is also one of the most difficult. Good fighting and puzzle skills are the only things keeping Pac-Man from being obliterated by Erwin's minions, as this level has plenty to offer in both areas. After completing this level, you'll find that the last three levels are a bit easier.