 In response to this situation , a small , D @-@ shaped stone fortification was built to protect Fowey Harbour in Cornwall , then an important centre for trade . The harbour was reached through the Fowey estuary , which the local town had protected in the previous century with two blockhouses positioned along the river 's edge — the Fowey and Polruan blockhouses — and a boom chain strung between them . The new castle replaced these and was located high on the headland overlooking the entrance to the estuary itself , St Catherine 's Point , from which it took its name . 
