Inchtuthil is the site of a Roman legionary fortress situated on a natural platform overlooking the north bank of the River Tay southwest of Blairgowrie, Perth and Kinross, Scotland (Roman Caledonia). It was built in AD 82 or 83 as the advance headquarters for the forces of governor Gnaeus Julius Agricola in his campaign against the Caledonian tribes. Positioned at the head of one of the main routes in and out of the Scottish Highlands, it was occupied by Legion XX Valeria Victrix and covered a total area of . Construction of the large fortress would have taken two or three seasons and a temporary camp was built nearby to house and protect the soldiers over the winter. Additional, smaller forts were built further north and south at the mouth of each nearby glen forming what are now referred to as the Glenblocker forts. Woolliscroft and Hoffmann argued that the Glenblocker forts, as well as others in Strathmore, such as Cardean and Stracathro, formed a uniform system composed of several elements, the forts and watchtowers on the Roman road of the Gask Ridge, the Glenblockers and the Strathmore forts. Inchtuthil as the largest military base would have functioned as the lynch-pin and the only site large enough to launch an invasion into the Highlands and beyond. Layout and garrison Unlike other legionary fortresses in Britain, Inchtuthil was not later built over and its layout was still largely preserved when Sir Ian Richmond excavated it between 1952 and 1965. It is therefore notable as the site which provides the only complete plan of a legionary fortress anywhere in the Roman empire. Its defences consisted of a turf rampart faced with stone, with an outside ditch and gatehouses on each side, following the standard Roman plan. The legion it accommodated would have numbered 5,400 at full strength, though there would have been additional specialist troops accompanying them. Facilities included a hospital (valetudinarium) that covered 5,000 square metres, a workshop covering 3,500 square metres and 64 barrack buildings. The timber walls of these buildings have been calculated to have had a total perimeter of seven miles (10 km). A headquarters building containing an aedes where the legion's colours and images of the emperor would have been kept has also been identified in the insula in the centre of the fortress. However, it was much smaller than would be expected for a legionary fortress,