Buttress
wooden or stone abutment for an apse or wall.

praetorian prefect (praefectus praetorio)
Commander of the emperor's bodyguard under the principate. During the regne of Constantine I the praetorian prefect becomes a dignitary responsible for the administrative unit called the prefecture, which was subdivided into dioceses. In 400 A.D. there were four such praetorian prefectures, of Oriens, of Illyricum, of Illyricum, Italia and Africa and of Gallia. The praetorian prefects were second only to the emperor. The praetorian prefect of Oriens was the mightiest among prefects. His office is for the last time mentioned in 680.

relieving arch
The relieving arches are built over opennings on the wall or, in case of fortifications, on the inside of the curtain wall, to help carry the weight of the elevation.

triumphal arch
(Rom.:) A structure in the shape of a monumental archway, built to celebrate the victory of a Roman general or Emperor.(Byz. Archit.) The arch formed above the Horaia Pyle (Royal Door), which frames the curve of the conch of the apse and separates the bema from the nave.

tympanum (lunette)
(Rom., Byz.) The arched panel (lunette) inside an arch or an arcosolium.

wallwalk (peridromos)
The platform at the foot of the parapet which made it easy to move quickly between towers and from which the defenders can safely fire over the parapet.