Hi Rhea!
I know EXACTLY what you mean, happens to me all the time again
Stand before the door of the dummy room inside the castle. Turn around and "cross the street". Don't run until you are in front of the wall, stop when you walk up that little diagonal plane (sorry, no better expression found). Then to your left and right should be the two entrances, one of which you are looking for. Maybe you need to look around a little bit, but it can only be a matter of 2-3 steps...
Hope that helps!
Never forget your towel, cheers
Philipp