The Invasion Of Wessex
Edmund and Egbert devoted most of their time to the building of the new fort, living very simply, and expended the whole of the revenues of the lands on the payment of the freemen and masons engaged upon the work. The Roman fort was a parallelogram, the sides being about 200 yards long, and the ends half that length. It was surrounded by two earthen banks with wide ditches. These were deepened considerably, and the slopes were cut down more sharply. The inner bank was widened until it was 15 feet across the top.

On this the wall was built. It was faced on both sides with square stones, the space between filled up with rubble and cement, the total thickness being 4 feet. The height of the wall was 8 feet, and at intervals of 30 yards apart towers were raised 10 feet above it, one of these being placed at either side of the entrance. Here the bank was cut away, and solid buttresses of masonry supported the high gates. The opening in the outer bank was not opposite to the gate in the inner, being fifty yards away, so that any who entered by it would have for that distance to follow the ditch between the two banks, exposed to the missiles of those on the wall before arriving at the inner gate.