foundations of a large colonnaded building are still to be seen below the present ground level. At Gloucester, Chichester, and Chester, the course of the four main streets has been little, if at all, disturbed, and their present meeting-place nearly represents the centre of the Roman city. The arrangements of the _forum_ of a Romano-British town have been made out very clearly by the excavations at Calleva Atrebatum (Silchester) in Hampshire.[11] It was a closed rectangle, entered by a gateway and surrounded by public buildings, in front of which were colonnades; one side at Silchester was occupied by a great basilica, which served the purposes of a hall of justice and mercantile exchange. [Illustration: Borcovicus] The stations on the Roman wall were of a more purely military character than the towns which have been mentioned. They have the general characteristic of a rectangular plan, with the angles rounded off, and with a gateway, flanked by guard-houses, in each of the four sides. In the two largest stations, however, Amboglanna (Birdoswald) and Cilurnum (Chesters), there were, in addition to the main gateways, two smaller gateways in the east and west walls respectively.[12] The walls of the stations are generally 5 feet thick, and are built, like the wall itself, of a core of cemented rubble, with facings of dressed stone. The main gateways have a double passage, divided by a longitudinal wall, which is pierced by a narrow passage in the centre. Their inner and outer openings were spanned by arches, and closed by gates which were hung on iron pivots fixed in the jamb of each opening next the wall. At Borcovicus (Housesteads) there was no dividing wall through the passage between the outer and inner openings of the gateway; but each of these openings is composed of two arches, divided by a square pier (15).[13] Each gateway had a stone sill, raised above the level of the stone pavement. The interior passage was flanked by rectangular guard-houses. The gateways of Borcovicus show interesting signs of reconstruction, which point to the fact that, not long after its construction, the station was seized by an enemy. At a subsequent time, its Roman occupants reduced the width of the gateways by walling up one half of the double openings. This was done apparently at different times, the east gateway bearing signs of being treated in this way at an earlier period than the others. The west gateway was walled up with great ingenuity. Of its outer entrances, the northern, and of its inner entrances, the southern, were blocked; so that a foe, choosing this face of the station for attack, had to press his way through an elbow-shaped, instead of a straight passage. [Illustration: Borcovicus; West Gateway] [Illustration: Pevensey]