Promontory Forts of Cornwall

Cornish promontory forts, commonly known in Cornwall as cliff castles, are coastal equivalents of the hill forts and Cornish "rounds" found on Cornish hilltops and slopes. Similar coastal forts are found on the north–west European seaboard, in Normandy, Brittany and around the coastlines of the British Isles, especially in Wales, Scotland and Ireland. Many are known in southwest England, particularly in Cornwall and its neighbouring county, Devon. Two have been identified immediately west of Cornwall, in the Isles of Scilly.

A promontory fort is a coastal headland, isolated from the mainland by a stone, turf or earthen rampart (univallate), or more than one (multivallate). Some promontory forts also have ditches. British promontory forts were constructed during the Iron Age, and remained in more-or-less continuous use into the early Roman period. Their function remains uncertain. They would have offered ready access to sea-routes but those built in particularly inhospitable settings may have had only occasional or seasonal use. Inland hillforts show signs of human habitation and other uses before and after the Roman era, but cliff castles were occupied sparsely, if at all. Some hill forts abandoned during or before the Roman era were reoccupied from the post-Roman to early medieval eras but in the same period cliff castles fell into disuse. Some were quarried for their building stone.

Treryn Dinas is one of the few Cornish promontory forts to have been systematically excavated. Archaeologists believe it might have been developed from a Bronze Age site of ceremonial, religious or social significance to the surrounding community. Possible ancient contexts and uses of Cornish cliff castles have been a subject for study and speculation by antiquarians such as William Borlase. Modern sources agree that cliff castles may have served principally as prestigious sites for religious ceremonies, trade and administration, and that their defensive capacity may have been a secondary function.

...
Wikipedia