Reviewing the typical characteristics of the special form of structure which has come to be known in recent years by the local northern name Broch, we see that it is a hollow circular tower of dry-built masonry, rarely more than 70 or less than 40 feet in its total diameter, and occasionally at least 50 feet high. Its circular wall, which may be from 9 to 20 feet thick, is carried up solid for about 10 feet, except where it is pierced by the entrance, or partially hollowed by the construction within its thickness of oblong chambers with rudely-vaulted roofs. Above this height the wall is carried up with a vacancy of about 3 feet wide between its exterior and interior portions. At every 5 or 6 feet of its height this vacancy is crossed by horizontal ranges of slabs inserted as ties between the outer and inner shells of the wall, so that their upper surfaces form a floor to the space above and their under surfaces become a roof to the space below. These spaces thus form horizontal galleries about 6 feet high and 3 feet wide, separated from each other vertically by the slabs of their floors and roofs. They run completely round the tower except that they are crossed successively by the stair which gives access to them. They are lighted by ranges of peculiarly-constructed windows placed vertically over each other, and all looking into the central area enclosed by the wall of the tower. This area or court varies from 20 to 45 feet in diameter. At various points of its interior circumference are placed the openings which give access to the chambers on the ground floor within the wall, and to the stair which ascends to the galleries. The only aperture on the outside of the tower is the doorway formed by the external opening of the tunnel-like passage through the wall which gives access to the central court. It is always on the ground level, square-headed, usually with slightly inclined sides,[75] 5 to 6 feet high, and rarely more than 3 feet wide, passing straight through the thickness of the wall, and thus varying from 9 to 18 feet in length. Some 4 feet or thereby within the outer end of the passage there is a rebate of the masonry faced with strong slabs inserted edgewise in the wall, and forming checks for a door, behind which are the bar-holes, and behind them the opening of a guard-chamber built in the thickness of the wall.