at least two levels of steps on the interior face, though there is no surviving indication of how the garrison reached the top. At the top of the wall the remains of medieval crenellations can still be seen, which probably replaced Roman originals. The D-shaped towers along the curtain wall are similar to those of several other Saxon Shore forts, although their placement is somewhat unusual. Because the fort was partly surrounded by marshes and water, which provided natural defences, the Romans economised by only building towers on the more vulnerable north-eastern and far western sectors. The towers were probably used to mount artillery weapons such as catapults and heavy crossbows. Ten towers still survive, though there may originally have been more before the loss of the south wall. Gates The Roman fort had two principal entrances, one on the east side and the other on the west, guarded by clusters of towers. The west gate covered the landward access via the causeway that linked Pevensey to the mainland. A ditch bisected the causeway, which led up to a rectangular gatehouse with a single arch around wide, with a D-shaped tower on each end from which archers could fire along the archway. The main entrance of the Saxon Shore fort at Portchester, built around the same period, had a very similar plan. Nothing now remains of the Roman gatehouse, which was replaced in the medieval period, while only a few stones are left of the medieval gatehouse. The east gate, wide, still stands; although what is visible now is principally medieval and 19th-century, the Roman original probably did not look much different. A postern gate was set into the north wall next to a section that has now collapsed. It was originally constructed in the form of a narrow curved passageway. Another postern gate may have been set into the collapsed south wall. These suggest that there may have been routes into the fort from across the marshes or access from a harbour, of which no trace remains. Interior The interior of the fort was artificially raised by the Romans, using earth dug from the foundation ditch, to bring it up to the level of the projecting step on the back of the wall. No evidence of significant buildings within the fort has been found by excavators. A number of Roman hearths are situated at regular intervals in the centre