Late in the 3rd century AD a chain of forts was built around the east and south coasts of Britain, for protection from invaders from the sea. Near Yarmouth is Burgh Castle (Gariannonum) which was built to protect the estuaries of the River Yare and River Waveney. Here the four sides of a large fortress have been preserved beside the River Waveney, and the walls of flint and brick are some 3m thick and stand 5m high. In places the walls have crumbled away, but the six remaining round bastions would have housed the ballistal or spring guns to repel invaders. Today Burgh Castle stands four miles inland but in Roman times there was a harbour just outside the western wall.

Further down the coast was Portus Adurni (Walton, near Felixstowe) and Othona (Bradwell, Essex), but a Roman document the 'Notilia Dignitatum' refers to another fort which was built about 32 miles from Caistor. This is most likely to be where the two Roman roads converge at Dunwich, and the fort was named Sitomagus ... but due to the continual erosion by the seas the fort would now be 11 miles from the coast, buried under the waters, if it still survives.

The Roman road nearest to Halesworth was this Caister/Dunwich road which follows the Stone Street to Broadway (near the Triple Plea public house - IP19 8QW ) and then carried across country to ford the Blyth River at Blyford. It then continued to the larger Roman settlement known to have existed at Wenhaston.

At Bungay it also connected with the camp at Tasburgh and ran via the Wainford Bridge on the outskirts of Bungay to Halesworth. It is thought that there was a military position at this point in Bungay, while a substantial building, perhaps a farmstead or villa, was built on the south side of Bungay.

There may have been a farmstead or small settlement at Halesworth, as a number of small villas or farm sites have been discovered on the higher ground of the river valley. From the large number of items collected by Gilbert Burroughes, it seems probable that there was a Roman site at Chediston. This appears to have been inhabited early in the Roman period in the 1st century AD, and is indicated by the early forms of pottery, in particular the Belgic ware which is of Claudian date, about 43-54 AD. There are indications that bronze casting was also taking place on the site, as numerous mould fragments have been found during the excavations. Among the items located was a lead balance from a steel-yard set of scales, a brass pin, two complete pots and other partly completed pieces of pottery in Belgic, Samian and Roman grey ware.