The fortifications of the Hellenistic and Roman city of Tocra are over 2 km long (including the sea-wall) and comprise a curtain wall up to 2 m wide flanked by 31 rectangular towers. Three main structural phases were noted in the survey carried out in 1966 by David Smith: (1) Hellenistic walls of isodomic ashlar, (2) later Hellenistic work of isodomic ashlar with bevelled edges, associated with the indented trace along the south rampart, and (3) an extensive rebuild of plain ashlar blocks including the towers and reconstruction to the East and West Gates, dateable, on the basis of Procopius, to the reign of Justinian. The general significance of the fortifications at Tocra is considered in the second part: these include the Hellenistic indented trace along the south side, later reinforced by towers in the sixth century AD. Also of wider importance was the use of an outer wall or proteichisma, and the pentagonal, pointed towers at the two main gates. Both these elements were unusual in Byzantine North Africa and they are discussed as part of the more general repertory of Byzantine fortifications. The unusual tower adjacent to the West Church is considered in the context of literary accounts. The article concludes by considering how the architecture and magnitude of the fortifications can allow a reassessment of the wider role of the city in the sixth and seventh century defences of Cyrenaica.