Chûn Castle is a large Iron Age hillfort (ringfort) near Penzance in Cornwall, England, United Kingdom. The fort was built about 2,500 years ago, and fell into disuse until the early centuries AD when it was possibly re-occupied to protect the nearby tin mines. It stands beside a prehistoric trackway that was formerly known as the Old St Ives Road and the Tinners’ Way. The name Chûn derives from Cornish: Chi an Woon. The area is now sometimes known as Chûn Downs. Nearby is Chûn Quoit.

Ringforts or ring forts are small circular fortified settlements built during the Bronze Age, Iron Age and early Middle Ages up to about the year 1000 AD. They are found in Northern Europe, especially in Ireland. There are also many in South Wales and in Cornwall, where they are called rounds. Ringforts come in many sizes and may be made of stone or earth. Earthen ringforts would have been marked by a circular rampart, often with a stakewall. Both stone and earthen ringforts would generally have had at least one building inside.