Banna (or Birdoswald Roman Fort) was a fort, towards the western end of Hadrian's Wall, in the Roman province of Britannia. Today the site is occupied by a former farm called Birdoswald. As of 2005, it is the only site on Hadrian's Wall at which significant occupation in the post-Roman period has been proven, and it is subject to a long-term archaeological programme under the directorship of Tony Wilmott.
It is one of the best preserved of the 16 forts along Hadrian's Wall. The fort is situated in a commanding position on a triangular spur of land bounded by cliffs to the south and east overlooking a broad meander of the River Irthing in Cumbria. In Roman times, the fort was known as Banna (Latin for "spur" or "tongue"), reflecting the geography of the site.
Excavations between 1987 and 1992 showed an unbroken sequence of occupation on the site of the fort granaries, running from the late Roman period until possibly 500AD. The granaries were replaced by two successive large timber halls, reminiscent of others found in many parts of Britain dating to the 5th and 6th centuries. Tony Wilmott (director of the excavations) has suggested that, after the end of Roman rule in Britain, the fort served as the power-base for a local warband descended from the late Roman garrison and possibly deriving legitimacy from their ancestors for several generations.
Extensive geophysical survey, both magnetometry and earth resistance survey, were conducted by TimeScape Surveys (Alan Biggins & David Taylor, 1999 & 2004) between 1997 -2001. These surveys established that the sub-surface remains in the fort were well preserved. The later surveys detected two vici (civilian settlements) of different characters on the eastern and western sides of the fort. It was these surveys which enabled Time Team to excavate in 2000, although substantial areas of the vicus still exist, some has been lost by erosion. The survey by TimeScape also determined the location of a bathhouse in the valley of the River Irthing.
An area between the fort and the escarpment was excavated by Channel 4's Time Team in January 2000. The excavation detected signs of an extra-mural settlement (vicus), but the area is liable to erosion and the majority of the vicus could have fallen over the cliffs.
Approximately 600 metres east of Birdoswald, at the foot of an escarpment, lies the remains of Willowford bridge. This carried Hadrian's Wall across the River Irthing. The westward movement of the river course over the centuries has left the east abutment of the bridge high and dry, while the west abutment has probably been destroyed by erosion. Nevertheless, the much-modified visible remains are highly impressive. Until 1996, these remains were not directly accessible from the fort, but they can now be reached by a footbridge that was lowered into position by an RAF Chinook helicopter.