aerial photograph Polruan Castle Cornwall England UK. Polruan Blockhouse was built by Henry VIII to defend the entrance to Fowey River alongside similar defenses on the other side of the river. A chain was laid across the river to another Chain Tower at Fowey so that the chain could be raised in times of danger to prevent ships sailing up the river.It was intended to prevent a repeat of the raid on the town in 1457. Polruan Blockhouse is one of only five Chain towers known in England with it and its opposite companion tower being the oldest. The tower became obsolete in the 16th Century when St Catherine's Castle was constructed, although it was reactivated during the English Civil War.