Dartmouth Castle, begun in 1481 in the shadow of its 14th century predecessor, stands guard over the entrance to the River Dart. Dartmouth Castle was one of the most advanced fortifications in England and is the first known castle in England to be designed with guns as its main armament. With Kingswear Castle, Dartmouth's twin across the water, they could cover the harbor entrance with cannon fire. A heavy chain could also be stretched across the estuary to the Kingswear side to prevent ships entering the harbor.

The term Castle, although formally embracing the whole area within the first curtain wall, is now usually restricted to the strong towers of 1481 and the gun platforms immediately to the north and south. The round tower was built first of large rubble, mostly limestone. Work stopped before the round tower was completed and a square tower was built beside it using slate. Ultimately, slate was used for the upper portions of the round tower when they were joined together. Due to its position on the cliffs, Dartmouth Castle sits on the rocks by the sea with the hill rising sharply behind it. If the castle were to be besieged, the attackers could easily pick off the defenders below them. The battlements were heightened on the landward side for protection against this threat.