Well, what I usually did on entering the castle was run to the upper right-hand corner and save the game immediately (usually, nobody was in the center area). Then I'd edge up towards the doorway while keeping an eye on the radar to see where the patrols were.
If something small was crossing the hall and nothing else was nearby, I'd step out and eliminate them. Otherwise, I'd sit tight.
The important thing is that no patrols are going through that left room, because you'll always have trouble. The six guards in there are fixed and you do have to take them out. It's easier if you don't have 20 more Rapax to worry about
I'm always set to run here so we waste no time getting into that room. Should we get caught, I restore and try again.
Nightowl2