None of the Newstead gates appear to have had more than a single entrance. It is probable that the passage was roofed with a barrel-vault, traces of this vaulting having been discovered at Birdoswald, and again at Housesteads. It made the gateway more secure, and enabled the chemin de rond to be carried across above it, so that the defenders could pass directly from one part of the wall to another without descending at the gateways. It can still be seen not only at Pompeii, but also in the gateways of the Aurelian wall at Rome; as well as in the fort of El Kastal on the Eastern Limes, where the remains of a second story above the gate are visible.[1] Thus bearing in our minds an impression of the exterior aspect of the fort, we may pass to the consideration of its interior plan and buildings.

1 Brunnow and Domaszewski, Die Provincia Arabia, vol ii. p. 95.