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.