Carlos Andrés B.
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At least 450 years old, the existence of this fort predates the founding of Boston.
The architecture corresponds to the military models of the time. It might not be as well preserved as other castle/forts in the Caribbean, but it certainly is worth your visit. I have 2 small children, and we found plenty to enjoy as a family in our 2 hour-long visit.
You can hire private tour guides at the entrance, but there is also plenty of information online to inform you as you search its many nooks and crannies.
It's well worth the 60 pesos entry fee.