When the fort was raised in the Iron Age, the inner wall ran twelve feet thick, ringed around a space two hundred feet across. Outside that wall ran a ditch forty-five feet wide and seven feet deep, and beyond the ditch an earthen rampart fifteen feet tall faced with stone. A stone-lined causeway crossed the ditch at the northwest entrance, where the ancient trackway from Penzance to Land's End passed through. In the centre of the enclosure stood a large circular stone building fifty feet across. Three Bronze Age ring cairns sit within the outer ramparts, older monuments folded into the new fort by builders who knew exactly what ground they were occupying. Most of the original stonework is gone now. In the nineteenth century, farmers and builders carted it away for walls and gateposts, the way they did everywhere in West Penwith. What survives is the geometry: rings within rings, a defensible heart at the centre of a defensible perimeter.

Caer Bran sits at a strategic point in the prehistoric tin economy. Tin and copper from the workings around what is now Pendeen and Botallack had to travel somewhere, and the natural ports were at Mount's Bay to the south and the Hayle estuary to the east. The fort overlooks at least three Iron Age settlements within a half-mile radius, including the famous Carn Euny village with its underground fogou half a mile to the west. The probable function of all this defensive engineering was to control and protect the metal trade. Cornish tin reached the Mediterranean by sea route long before Rome ruled Britain; Phoenician and Greek traders are known to have come for it. The men who lived inside Caer Bran's walls were probably less warriors than wardens, watching the routes the bronze came down, controlling who passed through to the sea.

The name connects Cornwall to a wider Celtic mythology. Bran the Blessed appears in the Welsh Mabinogion as a giant and a king of Britain, a figure with parallels to the Fisher King who guards the Holy Grail in Arthurian legend. The grail castle in those stories is called Corbenic, a name suspiciously close to Corben, the old French word for crow. The local Cornish folklore puts a granite pillar called Men Scryfa, a couple of miles to the north, at the centre of a battle story. The inscription on the stone reads rialobrani cunovali fili, Rialobranus son of Cunovalus — royal raven, son of the glorious prince. The tale goes that an invader attacked the prince at Penzance and drove him back to Caer Bran, where he made his stand. Bran was killed somewhere between Caer Bran and Chun Castle, and the Men Scryfa pillar was set up to mark his fall, the length of the stone supposed to match the height of the dead warrior.

Cornish legend gives Caer Bran a second life as a refuge. In the old stories the hill was a sanctuary from evil spirits, a place where the Pobel Vean, the Little People or faeries of Cornish belief, made their home. Neopagans still climb the hill on festival days. Walkers come for the panorama: Bartinney Castle a mile to the west, Carn Euny in the next field, Sancreed Beacon to the northeast. On a clear day you can stand inside the ring of Caer Bran and see most of Cornwall's prehistoric heart at once, fort, village, beacon, and chambered tomb arrayed across the moor like a single connected landscape. The Iron Age builders saw it that way too. They knew exactly which hill to climb.