The very latest spade-work on the Wall (undertaken by Messrs. Haverfield and Bosanquet in 1901) shows that the original wall and ditch ran through the midst of the great fortresses of Chesters and Birdoswald, which are now astride, so to speak, of the Wall; pointing to the conclusion that Severus rebuilt and enlarged them. In various places along the Wall itself the stones bear traces of mortar on their exterior face, showing that they have been used in some earlier work.
This is the number per lineam valli given in the 'Notitia.' Only twelve have been certainly identified. They are commonly known as "stations."
Antiquaries have given these structures the name of "mile-castles." They are usually some fifty feet square.