Just adding on to what Ram said,
Two of the four rooms (sorry can't remember which) that spawn skeletons have a large grey coloured brick that sticks out a little from the wall. From memory they are eye level, so just walk around each room until your screen shows up 'hidden button' or 'secret switch' or something. Also, the two buttons are in the rooms, not in the corridors that connect them. When pressed a wall will be removed and another room will be accessible, inside is a lever. Once both levers in the two hidden rooms have been pulled, the magical barrier dissapates.
Hope that helped, Good luck!
Tinman21