Castillo de San Marcos
Other sites of interest:
Starforts.com is the product of the patient genius of a fellow who works in magnificent anonymity. It is a pictorial listing of almost 300 starforts. There are galleries of forts that the author has visited. Full of information and plenty of humorous side-notes this is a most entertaining website. Visit it after you have bought a comfy chair......you will be there for hours!
JJ 's Wargames blog is written by Jonathan Jones it is concerned largely with Wargaming but, being British, he has far better access to forts in Spain. He has published fort visits similar to mine on his blog such a visit Ciudad Rodrigo, a much besieged fortress on the Spanish/Portuguese border and a visit to the battlefield of Salamanca as well as the Napoleonic Retreat to Corunna and a two part walk along Offa's Dyke in Jolly Old England .