Originally a fortress for the Spanish, Castillo de San Marcos was built completely using coquina, the only rock the Spanish could buy at the time. Luckily, coquina is formed from congealing of sea shells, and it remains a very solid rock. This was a solid fortress for the Spanish at war.
Right next to two Florida State Parks, visitors can take a self-guided tour using an app on their phones! Of course, there are rangers and volunteers in period dress to pose for pictures and answer questions.
Guided tours are available as well. You can catch a historic weapons demonstration every hour, weather permitting. If you check the castle’s app, you can see the next time the International Space Station will pass overhead!