So also at Carvoran and Birdoswald there is abundant confirmation of the _Notitia_ statement. The largest fort is Amboglanna, 5½ acres; the smallest is at Drumburgh, ¾ of an acre, an exceptionally small site. --------------------------------------------------------------------- {31} [Illustration: Fig. 2: COMPARATIVE PLANS OF MILE-CASTLE & TURRET] --------------------------------------------------------------------- The mile-castles or _castella_ were placed at the distance of one Roman mile, or seven furlongs, from each other. They vary in size, but are roughly about 60 feet by 50. The Wall forms their north {32} wall; their east and west walls are bonded into the Wall, so they were evidently built at the same time as the Wall. Their southern angles have been rounded off outside, though rectangular within. There has always been a massive gateway to the north and to the south, with a central road between, and inner buildings on either side of it. There were two Wall turrets between each pair of mile-castles. They were sentry-boxes, recessed into the great Wall, with walls 3 feet thick, and measuring about 12 feet by 10. The Romans always had a military way accompanying their fortifications. The Stanegate, made by Agricola, has a foundation of cobbles bedded in clay, and on that is a layer of cobbles or gravel, considerably raised in the middle. Kerbstones on either side mark its limits. It is now certain, through work done, that the paved military road which accompanies the Wall extended for its whole distance, running along between the Wall and the Vallum. It is about 18 feet wide, and can be very frequently recognized by its curved surface and stone kerbs, although it is grass-grown. The modern field-gates are very often placed upon it. {33} CHAPTER IV THE VALLUM The history of the Vallum in detail would appear to be as follows: The Emperor Hadrian decided that his new frontier should be defended by a chain of new forts, and that a great ditch--the Vallum-ditch--should be dug across the island, along the line of these forts, to mark the boundary of the Roman Province of Britain. This would be quite consistent with his usual policy of limiting the extent of the Roman Provinces in order to strengthen his hold on what it was most important to retain. By keeping to the south of the chain of forts, the ditch would come under their protection. That the ditch, and not the mounds, was the objective is pretty certain, for the ditch was made continuous at all costs, while the mounds were afterwards subject to trespass by the road, by quarries, by a mile-castle etc.