The towns of the Swahili coast of East Africa are widely acknowledged as the remains of a maritime society whose relationship with the ocean was fundamental to their economy and identity. Yet research that links the terrestrial environments of the towns to their adjacent maritime landscapes is rare, and urgently required in the light of marine erosional processes unmitigated by human actions. In the Kilwa archipelago of southern Tanzania, survey of the coastal foreshore has documented maritime architecture—particularly a series of coral-built causeways—that serve to link the medieval towns of this area to coastal resources and to expand the limits of the settlements themselves. This paper reports on survey recovering these causeways on Songo Mnara Island, putting the structures into context as part of the broader spatial manifestation of the island’s fourteenth–fifteenth century town. Several possible uses of the causeways are discussed, including functional explanations linked to the exploitation of oceanic resources, and more social reasons of territoriality and spatial demarcation.