The back of the postcard says "The Castillo de San Marco fortress dates from 1672 and has been occupied by Spanish, British and United States Military". That is quite an understatement. The fort was begun by the Spanish to protect the city following the attack by the English pirate Robert Searl. It is made of coquina, Spanish for 'little shells', a material similar to limestone. The British were fighting the Spanish for control of the Americas but unsuccessfully attacked the city and this fort a couple of times, eventually occupied it from 1763-84 when they translated and renamed the fort St Marks. After Florida became a US territory it was renamed Fort Marion after a military hero of the Revolutionary Wars. Then Florida proclaimed its independence in the civil war changing hands to the confederacy, then taken back by the union, all this without a shot being fired. It reverted to its original name in the 1942. For its long and eventful history Wikipedia has a rattling good tale here.
Despite the attacks the fort was never conquered in war and it looks as though it remains quite intact. It is now a National Monument, there are occasional military re-enactments here and looking at their calendar it seems most weekends a costumed demonstration of antique firearms.
The card travelled for 8 days and 4,113 miles (6,619Km) over the North Atlantic. Thank you Jim.