At a comparatively early stage in the art of Roman masonry in Britain the idea was conceived of protecting the enclosing wall of the fortress by means of projecting bastions and towers. In an early type represented in the Romano-British coast fortresses, of which this of Reculver is an excellent illustration, there were, as we have seen, no projections whether of walls, bastions, towers, or gates. Reliance was placed in the strength and solidity of the walls themselves, which were 8 feet in thickness. But the desirability of having some points from which the enemy could be attacked in flank whilst battering the wall soon became evident, and in other cases such as Richborough, Lymne, Pevensey, etc., we find that the fortress was furnished not only with massive walls, but also with strong angle-towers and bastions or towers at intervals by which the wall could be commanded and protected. These various works furnish an interesting series of illustrations of the progress made in the military architecture of the period. 5. RUTUPIAE [Illustration: FIG. 7. RUTUPIAE (RICHBOROUGH)] Now known as Richborough, situated about two miles north-north-west of Sandwich, was a station of great importance in the Roman period, being then, as Sandwich was subsequently for many years, the chief British port for travellers and traffic to and from the Continent. In shape Rutupiae was a rectangular parallelogram, with the greater length from east to west. Its walls, which were lofty and massive, enclosed an area of somewhat less than 6 acres. At each angle is, or was, a circular bastion 18 feet 6 inches in diameter, and square towers or bastions at intervals projected beyond the general face of the walls. A considerable part of the south-east corner, and the whole of the east wall have been destroyed by the falling of the cliff in the direction of the River Stour. The theory formerly propounded that the castrum had no eastern wall has been disproved by the careful examinations of Mr. G. E. Fox and other eminent antiquaries. These examinations have definitely shown that large fragments of the east wall have fallen down the cliff. It is certain that the castrum of Rutupiae as also those of Regulbium and Portus Lemanis, in spite of the doubt which has been expressed in each instance, had four walls.