Old San Juan "City Walls"

Construction began in 1634 after the Dutch sacked the city in 1625. It was essentially completed between 1638 and 1650, but minor work continued until 1678. The wall is composed of two 42-foot high parallel sandstone walls with the space between filled with sand.
It surrounds and fortifies the old city and once included five gates, the last one remaining is San Juan Gate 1790 near La Fortaleza. Some of the wall was dynamited near San Cristóbal Castle to allow the city to expand in the 1890. Portions of the wall have disintegrated due to hurricanes and poor maintenance over the years.