A short walk from the overhanging Wall brought me to AMBOGLANNA, at Birdoswald. This is the largest of all the forts, occupying 5½ acres as against the 5¼ of Cilurnum. Like Cilurnum it has six gateways, an extra and smaller gateway to east and west, lying to the south of the main eastern and western gateways.
Unlike Cilurnum, but like Borcovicium and Æsica, its northern wall is joined, at its rounded corners, by the Great Wall. The circumscribing ditch of the fort has been found underneath the Great Wall, as at Æsica, showing that the fort is earlier than the Wall.
An older fort is shown to have stood here, because the ditch which surrounded it has been traced, cutting across the present fort to the north portal of the eastern gateway, and this ditch can be traced again in one or two places westward.
The north gate of the original fort would be about on the site of the Principia of the present one. The excavations conducted by Professor Haverfield and Mr. and Mrs. T. H. Hodgson, between 1895 and 1900, prove that the fort was extended north and south at the same time, northwards over the ditch of the older fort, and southwards on to the Vallum. The south-west angle tower is where the north mound of the Vallum ought to be. The Vallum curves round the original fort, and makes a straight course east. To the west it has been entirely destroyed by a landslip.
Fig. 14.—Comparative Plans of Gateways. (After John Ward.) Scale: 30 feet to 1 inch.
It is a very steep fall to the Irthing on the south of the fort. Possibly the river has changed its course 100 yards since Roman times. Twenty years ago its course was changed 60 yards in a few hours at Underheugh (just under these cliffs to the east) by a flood.
The granary, just in front of the farm-house, has air-passages underneath, to keep the corn both dry and cool, as at Corstopitum. The gateways have as usual been built up in the later periods of the Roman occupation, and the floors raised. The north portal of the large eastern gateway has been blocked, and the west portal of the southern gateway. Both of these gateways are in good preservation, especially the eastern one, where one of the imposts is specially noticeable.
Inscriptions found at Birdoswald confirm the Notitia statement that the first cohort of the Dacians, styled the Ælian, was quartered here.