_Richborough Castle_, near Sandwich in Kent, may be cited as a veritable example of a Roman castle built in Britain, and is almost the only one remaining at the present day that preserves in any marked degree its original salient points. It is conjectured to have been erected in the time of the Emperor Severus, its mission being to protect the southern mouth of the great waterway which then separated the island of Thanet from the mainland, a similar office being performed by Reculvers at the northern entry. Three sides of the rectangle are still protected by the massive masonry walls which the Romans knew so well how to build; the fourth, or eastern side, where flowed the river Stour, possesses no visible defence, as it has been undermined and overthrown by the river-current. The northern boundary is 440 feet long, and the western 460. The walls, which vary in height from 12 to 30 feet, are about 12 feet thick and batter towards the top; they are beautifully faced with squared stone in horizontal courses similar to those seen at Segontium, the Roman station at Carnarvon; the core is composed of boulders from the neighbouring beach, embedded in mortar with courses of the usual Roman bonding tiles. In the centre of the area stood a temple and other buildings; the foundations of some of these are still in evidence. Whether the external walls were strengthened by the addition of square or circular towers of masonry, as at Porchester and Silchester, has not as yet been definitely determined. [Illustration: SECTION OF THE CITY DEFENCES OF VERULAMIUM (NEAR ST. ALBANS).] A common device in Roman castrametation was the berm or platform outside the surrounding wall, but immediately beneath it; in an attack upon the fortifications the assailants would be exposed to a plunging fire of missiles from the ramparts while descending the steep counterscarp of the ditch, to a raking discharge when ascending the slope of the scarp, and be entirely devoid of cover when crossing the berm, which was generally about 20 feet wide. Another advantage of the berm was that it placed the engines of the besiegers on the remote side of the ditch at a greater distance from the walls, and thereby lessened the effect of the missiles discharged from them. To still further modify the results of the latter upon the wall it was customary to bank up the earth upon the inner face to form a ramp, and this also lessened the effects of the rams of the besiegers. These features are shown in the foregoing diagrammatic section of the walls of Verulamium. (_b_) _The Saxon Period, c. 410-1066_