Oh, the "grand idea" was that it was the same path each time, but with brand new rooms with new monsters inserted into the path each time. You'd learn the mutations on the monster you met before ("oh, this is the room with the speedy blue ghosts, so I'll need to fight them in hallways") and meet the new monsters in new rooms to learn about them before they grew more powerful on future mutations.
Sadly I didn't get enough rooms or terrain variety in the game before the deadline (I promised myself to make all my own assets - foolish, I know) so it's not as apparent that the castle is growing, as opposed to a very repetitious procedural dungeon.