Returning to the Common Room late one evening from the kitchens by way of one of the hidden passageways, Harry asks one of the portraits he's gotten to know well if there were more secret passageways that nobody remembers.
"You know, there are eleven ways out of the castle. I believe you know of four of them. Three may have been closed or collapsed." the portrait replied.
"Eleven! I've heard of seven. And yes, four are unusable." Harry exclaimed.
"Well, there's the Shrieking Shack one, the one to that chocolate store in town, the one to the back side of the lake, the troop entrance on the other side of the mountain, the chamber one I think you found, and..."
"Wait. the troop entrance? That one I haven't heard of. Or at least by that name."
"Yes. During war times when the castle was under assault, troops and supplies came through from the other side of the mountain through a hidden passage through the old storage rooms, weapons room and the sleeping quarters. If you go to the second floor, there's a large closet where the wall moves away, down a long flight of stairs..."
"A weapons room?"
"Oh, yes. That's where the troops lived and trained. There's a whole compound set up for 500 men to live and train."
So what do you want to learn about Hogwarts castle? I'm sure there's plenty more to discover. You just have to write about them.
*** A Late Note: This Challenge is based on an original suggestion by Lazy Lazuli. Thanks JP.