The Dart estuary is a large 'ria' (coastel inlet). It is popular for sailing. The entrance to the river from the sea is rocky with cliffs either side. On the East side Kingswear Castle sits very close to the water's edge, and on the west side Dartmouth Castle is built on a rocky promontory at sea level. The castles once operated a defensive chain across the estuary. It was raised at dusk to destroy enemy ships attempting to attack the harbour. The remains of the operating mechanisms for the chain are still visible in Dartmouth castle.