Mural towers vary much in form and size, but are more usually round, or half-round, or half-round with prolonged sides, than square. Now and then they are polygonal, as at Stokesay, Warwick, Cardiff, and the Bishop’s Gatehouse at Llandaff. Stokesay is of the thirteenth century, but usually these towers are later. Mural towers have seldom much internal projection. In Roman works they are usually solid, but in later castles they are sometimes open in the rear or gorge to prevent their being held when taken by the assailants. The towers of Cologne and Avignon are so open, and at the Tower the Byward and Traitors’ Gate towers are closed only with a wooden brattice. The annexed drawing shows the back view of Traitors’ Gate, with the large arch over the water and the timber-work above it. Where there was an outer ward and a second wall, this was considerably lower than the inner wall, and commanded from it; and the mural towers often became mere bastions, not rising above the curtains. The second ward, besides being too narrow to allow of troops being massed or machines posted for the attack on the inner wall, was traversed by cross walls with gates, so as to isolate any body of assailants who had breached the outer wall. Such cross walls may be seen at Caerphilly and at the Tower. The barbican was sometimes a mere walled space attached in front of the gateway, as at Carlisle, Alnwick, Richmond, or the Bars at York. Sometimes it was a _tête du pont_, detached from the main work, and posted at the end of the bridge, upon the counterscarp of the ditch, as at Hawarden, Ledes, and Goderich. At Scarborough it is placed at the outer end of the long raised causeway, which, again, is broken by two drawbridges falling from a low central tower. At Helmsley, as at Caerphilly, the barbican partakes of the character of a horn-work. The Tower barbican, called the Spurgate, is a regular low gatehouse, with flanking drum towers or bastions covering the head of the bridge, and itself protected by a loop from the main ditch. At Bridgenorth the barbican contained a kitchen, ordered to be repaired, 17 Hen. III. At Canterbury the barbican, _temp._ Ed. II., was used as a prison. [Illustration] Spelman defines a barbican or “ante-murale” as “munimen a fronte castri aliter ante murale dictum; etiam foramen in urbium castrorumque mœniis ad trajicienda missilia.” A palisaded embrasure in front of the barbican was known as the “barriers.” They are well represented at the Tower by the stockade covering the entrance.