The Cuartel de Ballajá is the largest building in Old San Juan — a vast three-storey neoclassical military barracks finished in 1864 to house Spanish troops. The block surrounds a central parade ground; long arcaded galleries run around all four sides of the interior courtyard at two levels.
The Cuartel was the last major building completed under Spanish rule before the 1898 American occupation. After serving as a US military hospital and then a school, it has been restored as a cultural complex — the Museo de las Américas occupies most of the upper floors. Even if you skip the museum, walk through the courtyard; it is the most impressive interior architectural space in the old city.