La Fortaleza was built in the early 1500’s as the first fortification in San Juan Harbor. Built to defend against invaders, the fort was only successfully conquered twice, once by the British in 1533 and once by the Dutch. In 1625 the Dutch invaded and burnt the city including the fort. Today the fort is the residence and working offices of the Governor of Puerto Rico and is the oldest functioning Executive mansion in the western hemisphere.
Fortaleza is a functioning government building and it’s subject to closures without warning. It is advised to call beforehand to get the itinerary of the day
It was certainly intended as such when it was completed in 1540 as part of a massive construction effort to secure the island’s defenses. It didn’t do so well, though, falling to the Earl of Cumberland in 1598 and the Dutch Commander Boudewyn Hendrick in 1625.