Castillo San Felipe del Morro.

Guarding the entrance to San Juan Bay, Castillo San Felipe del Morro is a colossal 16th-century citadel that stands as a masterclass in Spanish military engineering. Locally known as El Morro, this six-level fortress was built over centuries, starting in 1539, resulting in a maze of stone ramps, dark tunnels, and barracks shielded by walls up to 18 feet thick. Visitors can explore historic dungeons, stand inside the iconic domed garitas (sentry boxes), and spot the internal lighthouse. Outside, the dramatic coastal setting on a rocky headland offers sweeping views of the Atlantic. The experience is softened by the massive, wind-swept grassy esplanade leading up to the fort, where locals and travelers regularly gather to fly kites against the backdrop of the ocean.