Besides these openings, there are at the same level three mural chambers and a staircase. The principal chamber occupies the south-west angle, and is in plan a right angle with two limbs, like the capital letter L. That in the west wall is 5 feet 6 inches broad by 14 feet long; that in the south wall, 4 feet 10 inches broad by 23 feet long; but as each is measured over the breadth of the other, the total length of the chamber measured on the outer wall is 37 feet and measured along the inner wall only 26 feet 8 inches. The chamber is partly lined with chalk ashlar and partly with rubble, and the vault, barrel and round-headed, is of rubble. The vault springs from a plain Norman abacus. The height to the springing is about 7 feet. The outer wall of this chamber has been lined throughout with an arcade, originally of ten arches, of which six remain quite perfect, and of most of the others there are traces. The arcade is of late Norman work, the piers delicate, the caps very highly carved, the arches round-headed. The whole is recessed in the wall, reaches to the springing of the vault, and rests upon a low plinth or dado. There is a loop in the west wall near the north end of the chamber and another in its south end in the south wall. There is also a third and longer and lower loop at the other end of the south limb, close to, and on the right of, the priest as he stood before the altar, the place of which at the east end is marked by a bench or step in the wall. One original door was in the west limb, close to the main entrance. In King’s time it was perfect, and was round-headed, 2 feet 4 inches wide and 7 feet 7 inches high, but it has since been broken away. There is a larger, rude opening in the south limb, which may represent the place of another door. That this singular and highly-ornate chamber was originally an oratory is evident, both from the care bestowed upon it, from the traces of an altar in the east wall, and from the window next the altar.