Castillo de San Miguel is a squat 16th-century fortress built to guard the old harbour from pirate raids. After the eruption destroyed that harbour, the castle lost its original job but survived as one of the best markers of Garachico's former importance.
The little museum inside covers the town, the eruption, and the local coast. The tower is more useful than the labels: from above you can read the curve of the lava shore and see how close the sea once came to the old streets.