The north, south and west sides followed the usual pattern for a Roman camp of straight ditches and ramparts.
However, on the eastern side the defences bulge out around a circular structure with a diameter of 32 m (105 ft).
The sand and gravel subsoil had been dug out to a depth of 600 to 900 mm (24 to 35 in) and the area surrounded with a timber stockade.

This ring, the only known "gyrus" in the Roman Empire, may have been used for training horses.
The gyrus was added to the fort during its second period of occupation and its construction caused significant disruption to the fort.
 Having the gyrus within the fort affects not just the wall which curves to accommodate the structure deviating from the Roman playing card shape pattern but also the layout of the fort which is significantly different from the standard layout.
 This makes the fort unique not just in Britain but also in the Roman Empire.
The north, south and west sides followed the usual pattern for a Roman camp of straight ditches and ramparts. However, on the eastern side the defences bulge out around a circular structure with a diameter of 32 m (105 ft). The sand and gravel subsoil had been dug out to a depth of 600 to 900 mm (24 to 35 in) and the area surrounded with a timber stockade.
This ring, the only known "gyrus" in the Roman Empire, may have been used for training horses. The gyrus was added to the fort during its second period of occupation and its construction caused significant disruption to the fort.  Having the gyrus within the fort affects not just the wall which curves to accommodate the structure deviating from the Roman playing card shape pattern but also the layout of the fort which is significantly different from the standard layout.  This makes the fort unique not just in Britain but also in the Roman Empire.
Lunt Roman Fort