Fig. 9.—Fortification Turrets from Trajan's Column. (After John Ward.)
A guard-chamber was always set on either side of each main gateway; there was a turret just within each rounded corner of the fort, and intermediate turrets were set along the walls.
The appearance of these turrets may be surmised from the fortification-turrets shown on Trajan's column.
Chambers above gateways are also shown on Trajan's column, which suggest to the mind's eye a possible reconstruction of the single gateways of the Wall-forts.
Fig. 10.—Fortification Gates from Trajan's Column. (After John Ward.)
It is generally accepted that the twin flanking-towers of the double fort-gateways, and also the Wall turrets, were carried up from one and a half times to twice the height of the wall (whether fort-wall or Great Wall). The rampart-walk is reckoned to have run at a height of from 13 to 15 feet from the ground, so that it would pass over a gate quite horizontally, and without steps. It would be continued right through the towers and turrets, passing through doorways in their side walls, and across the floor of their upper storey.
Mr. John Ward (in Romano-British Buildings and Earthworks, p. 70) calls attention also to a gate figured on a mosaic in the Avignon Museum, as suggesting the possible character of the double gates on the Wall-line.