I think it depends on what House you're in, or perhaps what mood the castle is in. I've heard that walls can pop up, blocking off the stairs, or sometimes the stairs just disappear, or turn into a steep hill so that anyone climbing them slides down. But I've never heard that about girls going into boys dormitories, and -

Well, Hermione goes to Harry's dorm. I know that she does. So it must not apply to girls.

From,
Hydra