San Pedro de la Roca Castle, Santiago de Cuba

Commercial and political rivalries in the Caribbean region in the 17th century resulted in the construction of this massive series of fortifications on a rocky promontory, built to protect the important port of Santiago. This intricate complex of forts, magazines, bastions and batteries is the most complete, best-preserved example of Spanish-American military architecture, based on Italian and Renaissance design principles. [source]

I haven’t been here (yet).