Chun is a ringfort, a round walled enclosure rather than the angular hillforts of southern England. The plan is concentric: an outer wall and ditch ring an inner wall and ditch, and the two entrances are offset so that an attacker who breaks through the outer gate finds himself trapped in a curving corridor with defenders on either side. The original inner wall was twelve feet thick. The well sits within the inner enclosure, originally reached by a stone stairway descending to the water. Even in the long dry summers, when the wells in the village below ran empty, Chun's well kept its water. Locals were still drawing from it as recently as the 1940s, some for cooking and washing and some for older reasons: the well was reputed to grant perpetual youth, and even in the twentieth century there were people in Cornwall who preferred to hedge their bets.

Chun sits beside a prehistoric trackway, an ancient road that ran the length of the peninsula and was variously called the Old St Ives Road and the Tinners' Way. The fort overlooks the only land route into West Penwith from the south, and the Celtic Sea opens out below it to the north. The position is not accidental. The richest tin and copper country in Iron Age Britain lay just over the next ridge, around what is now Pendeen and Geevor and Botallack, and Chun was almost certainly built to control and protect the metal that moved down this trackway. Iron and tin slags found within the walls confirm that smelting happened on site. The fort fell into disuse around the first century AD, then was reoccupied and modified in later centuries, possibly into the Roman period and as late as the sixth century. The metal trade made and unmade it. As long as Cornish tin mattered to the world, somebody held this hill.

When archaeologists dug here in 1895 and again in the 1920s and 1930, they found pottery dating to the fourth century BC, with shapes and decoration closely matching contemporary work from Brittany across the Celtic Sea. The connections of Iron Age Cornwall ran south and east as much as east into England. But the fort's foundation may be older still. Chun Quoit, just 800 feet to the west, was raised in the Neolithic around 2400 BC, two thousand years before the ringfort. The builders of Chun chose a site that had already been sacred ground for two millennia, a hilltop where the dead had been buried and the ancestors had gathered. It would not be the only time in Cornish prehistory that an Iron Age people built their new walls on much older bones.

The slow destruction of Chun in the nineteenth century is its own story. In 1883 the freehold of part of the castle ground came up for auction as a tenement called Little Bosullow, and the sale notice openly advertised the large quantities of good building stone available on the site. The masons of Penzance and Madron worked the ruins like a quarry for decades, carting granite blocks down the trackway for new houses and walls in the growing towns. The 1886 annual meeting of the Penzance Natural History and Antiquarian Society recorded its alarm and asked that the looting stop. It did, eventually, but not before two-thirds of the wall height was gone. Today neopagans climb up here on the solstices and the cross-quarter days, leaving small offerings at the well. English Heritage protects the site. The walls have stopped getting shorter, and the water still rises clear, as it has for twenty-five centuries.