There is a door on the second floor that will not open. I see no levers or switches, it is in the middle of a hallway,any ideas? Thanks

There is one room with a pillar in the center, with entrances at all four points. The corners of the room have no floor, though. If you drop down one of these areas, you will end up in a room where you can exit, but not get back in. This could be the one you mean. I don't believe there was anything vital in the treasure chest.