Merlin's Cave is on the right side of the rock at Tintagel (if you are standing facing the sea). It is very atmospheric, and one might indeed imagine Merlin approaching, with his staff held up to light up the darkness of the cave.

The steps down to the cave have been washed away and are blocked off. The only way down today is to clamber down the rocks to the beach. The cave fills with water at high tide, but has a sandy floor and is explorable at low tide. The cave goes all the way through the rock, and there is a smaller cave off it that can be entered on the southern side.