The gyrus is a large circular structure on the east side of the fort. This feature is unparalleled in Britain.

The ring is 34.06m in diameter with a funnelled entrance and double gate, probably indicating that the structure was used for livestock of some sort.

Archaeological excavations uncovered finds associated with cavalry. This strongly suggests that the gyrus was once a cavalry training arena where the soldiers stationed in the base trained horses for combat. The surface of the arena was kept smooth and level and was covered in sand. The Greek historian Xenophon referred to such a structure as a 'gyrus', but there is no equivalent Latin word.

Though the gyrus was not part of the initial fort design, it was obviously an important part of the fort. When the structure was added, two barrack blocks were shortened and one was moved to accommodate it. Once built the arena remained a constant feature while the rest of the camp continued to be altered and developed.

The gyrus was the last part of the fort to be dismantled in AD78. It is thought that the gyrus was used as a stockade for soldiers and supplies when the fort was being taken apart.

The reconstruction of the Gyrus was completed in 1977. 18 men from 31 Base Squadron working 6 hours a day took 10 days to complete it.

Sleeper beams were first set into the perimeter trench and then uprights were set into these. A fence was constructed all the way around the circular arena with a double gate built at the each end of the entrance.