Tintagel Island and Castle were associated with the legend of King Arthur. This theory is now out of vogue. Many new ruined dwellings have been discovered on this bleak island. It was possibly a fortified port when it was hard to distinguish between traders and raiders. Who knows - but it is a dramatic and mysterious place. The fort is gradually falling into the sea due to erosion.
One tip, you can avoid parking fees adjacent to the island as follows. When you come into the town, take a left turn down "Vicarage Hill" lane just past the old post office. Turn right into "Church Hill" lane. It takes you to a free car park alongside Tintagel Parish Church. This site has a good view of the coast. Take the path to the north along the coast for a short scenic walk to the entrance to the island site.