Back in the day, Risley had lots of little secret passageways and cubbyholes. Then, (some as recently as 1999), they got sealed up by uninquisitive minds that didn't want to know.

This is one of them. If you look in the wood paneling, you can see a crack around where the door used to be, before the wooden crossbeams were put over it. The passage skirts all the way around the Great Hall.

Great for evesdropping on visiting dignitaries, no?