The toilet in the living room is set into the thick wall and has a door attached to it for privacy and has recess cut out of the wall for a lamp or candle.
This toilet along with the one in the bedroom are just a drop shaft directly to the ground. The material that would have been deposited down this shaft may have been left to rot at the bottom or removed by servants and disposed of in the marshland that surrounded the castle which is now long gone.
The toilet support is completely made of stone and would have either a slab of stone along the top with a hole in it to sit on or a piece of wood for a bit more comfort. Over the years whatever covered this toilet has been lost but it is more than likely it was made of wood.
Nowadays there is a sheet of thick glass covering the hole that was placed there in the early 1970 for when the castle opened to the public.