El Fortín Conde de Mirasol
On the island's northern coast, above Isabel Segunda, is the last military structure built by the Spanish in the New World. It was erected in 1840 at the order of Count Mirasol, then governor of Puerto Rico. Although it's tiny, it took more than a decade to complete, which meant Mirasol had to repeatedly ask for more money. (Queen Isabel, on being petitioned yet again, asked Mirasol whether the walls were made of gold.) The hilltop fort helped solidify Spanish control of the area, keeping British, French, Dutch, and Danish colonists away and dissuading pirates from attacking Isabel Segunda. In 1991, after sitting empty for several decades, it was transformed into a museum with a good array of artifacts from the Taíno and other cultures that thrived on this and nearby islands before the arrival of the Spanish. It also has an impressive collection of small arms, plus exhibits on the island's years as a sugar plantation and its occupation by the U.S. Navy. On occasion, it presents temporary exhibitions of works by contemporary artists.
