artmouth Castle is one of a pair of castles that guard the entrance of the wide estuary of the River Dart at Dartmouth on the south coast of Devon. The other smaller castle in the pair is the Kingswear Castle. Pendennis and St. Mawes castles are a similar arrangement of castles that can be found at the Fal estuary at Falmouth. The estuary at Dartmouth is quite narrow at the sea but widens and goes far inland and is a perfect location to anchor a fleet of ships. For any invader of Britain this natural harbour would be a great prize to hold. The narrowness of the estuary at Dartmouth made is easy to construct two castles that could guard the river with canons.

Kingswear Castle is located low on the rocks on the eastern is of the habour entrance and consists of a small rectangular block and a later, smaller round tower. Kingswear was adapted by King Henry VIII to house canons. Over time this smaller castle became less important. Dartmounth Castle is the larger of the two and was orginally build in the late fourteenth century. It was improved upon in around 1481 to house guns that could overlook the entrance to Dartmouth harbour. It consists of a large square tower with a lookout turret on the top. Beside the square tower is a round tower from which a chain was attached to the round tower at Kingswear on the other side of the harbour entrance. This chain was raised during the night to guard against enemy ships entering unseen.

The castles were held by the the Royalist supporters of King Charles I during the English Civil War but were captured by Sir Thomas Fairfax in 1646.