(To which version of the game does this apply?  Untested.)

This material was exported from the Codebook 4:
http://www.codebook.pp.se/

Dark Castle : Cheats

Flying

Just as you start to go up or down a set of stairs, hold down the 'duck' key.  At the end of the staircase, you'll be walking approx. Three feet off the ground, in game terms.  This requires some careful timing, so it may take a few tries.  Be careful, though; you still can't walk off ledges safely...  You can avoid rats and still pick stuff up, however.  Enjoy.
