Veracruz is, above all else, a shipping port. Our hotel looked out over the main port area, and one day after snapping the above picture we happened to see the same ship leaving from our hotel window.
The stone fortress you see in the picture below is San Juan de Ulúa.
Originally an island, and now located in, and connected to the rest of the port structures, San Juan de Ulúa was originally built in the 16th century as a defensive structure by the Spanish.
Many of the walls of the fortress are composed primarily of coral, as you can see in the section below.
For a century or so, the fortress served as a prison, primarily for political prisoners. Its thick walls and isolated location provided a perfect site.
Small stalactites have formed on the roofs of many of the rooms.
We spent several hours in fascinated exploration of the place.
It is definitely worth a visit. Our guidebook said that you could get there by bus, but there no longer seems to be one. Instead you need to catch a cab - the best place to leave from is Plaza de la Republica (just east of the Zócalo).