Surrounded by a deep moat and perched on a high mound, the huge circular keep of this castle, built at the turn of the 14th century, survives in good condition.
One of a pair of defensive structures, the other being on the far side of the river mouth in Fowey. The blockhouses were linked by a huge metal chain which could be raised to prevent ships entering the harbour during an attack. Built in 1380 primarily to defend from French and Spanish.
The 16th century Old Blockhouse is located at the southern end of Old Grimsby harbour on Tresco. It was built in the reign of Henry VIII as part of a series of coastal defences to protect against invasion from the French.