First Abandonment or Gap in Occupation: Third Phase
The traditional view is that the first period of Roman occupation lasted from 80 to 105 AD. The Romans gave up trying to have a Tay-Forth or Clyde-Forth frontier line of forts. "In the early years of the second century the costs or risks of maintaining the outposts (in South Scotland) appear to have exceeded the advantages, and they were abandoned. For nearly forty years thereafter the lower isthmus (of the province of Britannia) formed the north-western frontier of the Empire, and the tribes of Scotland were left to pursue their respective ends without the hindrance or help of a resident garrison." G S Maxwell, 1989.
The Emperor Hadrian's visit to Britannia in 121-122 AD resulted in the building of the Hadrian's Wall frontier. Again the traditional view is that Trimontium was re-occupied about 140AD and became a support centre to the rear of the (Forth-Clyde) Antonine Wall, when Hadrian's successor Antoninus Pius brought an army back into Scotland. Trimontium may well have been re-occupied some time before that as an outpost fort, North of Hadrian's Wall.