The Marching Camps and the Fort: First Phase - The Agricolan Fort
The earliest Roman remains on the site are a series (eight at the last count) of large marching camps (the ditches still visible after 2,000 years as dark lines in aerial photos). They represent temporary stopping places for tented armies on the march ie for the first garrison before they completed the fort or, after it was occupied, for visiting troops passing up or down the line, who could not be accommodated in the fort itself. They are of varying sizes, two being 40 and 50 acres in extent, and one being called by Curle 'The Great Camp'. Trimontium may have been a gathering place for armies eg under the Emperor Septimius Severus in 208-10 when he campaigned in the North of Scotland and may have wintered at Cramond.
There are many periods or phases of fort construction at Trimontium. The first in 80 AD probably built by the Ninth Legion from York ( not 'lost', according to the latest evidence) during Agricola's northern campaign leading up to his victory at Mons Graupius (in Aberdeenshire?) in 83 AD, was about 10.5 acres in extent, contained wooden buildings, and was defended by a rampart of earth only, built up on a foundation of cobbles, and with two V-shaped ditches, 9' wide and 3' deep approx, in front. In shape it was an 'irregular' fort in that it departed from the standard playing-card shape layout. The lines of the rampart in each quarter are staggered, so that people approaching the gates in each side must do so at an angle, thus exposing themselves to side fire. Other Agricolan forts in Scotland display similar characteristics, but the overall idea seems to be exceptional (the work of one engineer?) and examples are rare. See outline drawing. The West Annexe, an enclosure defended by two ditches, seems to have been the first extra-mural development.
It is suggested that the ala Petriana, the biggest in the country, provided the cavalry wing stationed at the fort at this time.