From De Aedificiis by Procopius 
“Beyond Athyras is a certain place which the inhabitants call Episcopia. The Emperor Justinian, perceiving that this lay exposed to the assaults of the enemy, and that a large expanse of country here was altogether unguarded, since no stronghold at all existed, built a fortress in that place; and he built the towers there, not in the customary manner, but as follows. At regular intervals a structure is built out from the circuit-wall, very narrow at first, but finally spreading out to a great breadth; on this in each case a tower was erected. Thus it is impossible for the enemy to get close to the wall anywhere, because when they get into a precarious position between the towers they are easily shot at from both sides and from above by the guards there and are destroyed. The gates too he did not place in the customary position between the towers, but at an angle, in the narrow part of the projection which runs out from the wall, where they could not be seen by the enemy but were masked behind the towers. In that place Theodore, a very clever man who held the office of silentiarius, was of service to the Emperor. Thus were these fortifications built. And it is proper, proceeding thence to the long walls, to explain them briefly.”