On the city wall.
When Duncan took the initiative to ask for control of the ballista, the Duke of Dingtiger agreed without hesitation.
It was well known that the Roman legion excelled at using ballistae.
Duncan's appearance clearly showed he was from the regular army, and his request for control of the ballista surely meant he had some confidence.
The Duke of Dingtiger had gathered nearly all the nearby farmers; counting several hundred miscellaneous soldiers, they amassed a force of fifteen to sixteen hundred people.
The Isle of Britain was merely a province of the empire, so the scale of battle would be much smaller, with engagements typically involving regular armies of a thousand. Adding in vassal armies, mercenaries, and farmers occasionally allowed the claim of tens of thousands.
In the early period when the British province was abandoned, a single legion of the empire could suppress the surrounding barbarian races.