This is Fort Charles, a ruin sited just half a mile or so to the south of the town of Salcombe on the Kingsbridge Estuary in Devon.
This fort was one of the many built along the south coast of England by King Henry VIII to protect the most important ports from raids by the French and the Spanish. It was the last of the forts occupied by royalist forces to surrender towards the end of the English Civil War. A particular royalist stronghold, it was ruined on the orders of parliament as they deemed it too dangerous to remain in a usable condition.
Fort Charles is a Grade II listed building and is a scheduled ancient monument.