Castle Beach sits directly below the castle headland with the ruins as a backdrop and St Catherine's Island sitting in the water just offshore. At low tide the beach widens dramatically and you can walk out towards the island causeway. The combination of sand, ruins and tidal island is pretty hard to beat.
It's slightly less accessible than North Beach which keeps it marginally quieter. The views from the headland above are even better, walk up via the castle path and you'll see all five beaches at once.