Pendennis castle, erected between 1540 and 1545, was the most westerly of a chain of coastal defences erected by Henry VIII in response to the threat of French and Spanish invasion. The original Henrician defences have been significantly enhanced through the years, notably in the Elizabethan period and in the 18th and 19th centuries. The castle was used as a barracks, storehouse and sergeant's mess in the late 19th and early 20th centuries. During both the First and the Second World Wars, Pendennis castle became part of Britain's coastal defence system. The site now houses a permanent exhibition on the castle.
The Henrician castle was constructed on land leased from the Killigrews of Arwennack. Construction started in 1540 and a circular keep or tower (nearly 17.5 metres in diameter) surrounded by a low polygonal chemise or curtain wall was built. The name 'Pendynas' suggests an ancient fortification already existed on the site and a plan of about 1540 seems to indicate the former presence of a cliff castle. Together with its sister castle at St Mawes, Pendennis defended the approaches to Carrick Roads, one of the largest natural harbours in the country.
In 1597, late in the reign of Queen Elizabeth I, the fortifications were extended with the inclusion of ramparts and bastions to guard against Spanish incursions. The castle was again strengthened prior to the Civil War. In 1646, it was the last Royalist position in the West of England and a Royalist garrison withstood a 5 month siege from Parliamentarian forces before surrendering. Improvements were made to the outer defences in the 18th and 19th centuries, and the castle was used as a barracks up to the time of the First World War. During the First and Second World Wars a number of coastal batteries were constructed on the site as part of Britain's coastal defence system (see associated monuments). The site is now run by English Heritage and there is a permanent exhibition on the castle.