My thought was to have them save when they enter, do whatever they do in the room - might be quick - then next room save again to see if they stay the same, are cured, or worsen. The effect lasts until they leave the room, and enter the next one - check again.
Good point on speed, but I'm not sure I follow the mechanic you suggest. Can you elaborate on what you mean by temporary? I'm game for some help with this, but love the idea that they are constantly penalized, in some way, for being in the castle or at least constantly reminded that they are in a place they should not visit.
Now, I could have them suffer this, but locate a way to neutralize the effect in the castle. Perhaps once they activate the legacy item they bring in (the sunsword or symbol).