I found a bug!
I made it to the final stage of the level. I found all four hammers for opening the shrines, but I cannot return to the chapel again, after I opened it with the special key and closed the door by hand to prevent noise issues with the undead. I've been in the chapel earlier (there's a key and without it's not possible to enter another area).
So now I stand in front of the closed chapel door and have to restart the level, what I won't do since I always save my gamestats via quicksave. There's no older state I can recall.. so that's it for me.
Thanks anyway for the really great experience of a huge detailed level!