I absolutely love this national monument. I've been here a few times before (while growing up, and about 5 years prior) but this trip was the best one. Chris and I walked through every inch of the fort and read and watched all the information available- it's fascinating. The massive fort was built to protect the city of St. Augustine, sitting alongside the gorgeous Matanzas river. The original construction started in 1672, finishing in 1695.
There are all sorts of rooms to go into once inside the fort, including a very tiny storage room that you have to literally crawl to get inside. It's lit, but still dark, damp, and creepy. You can almost feel the weight of the heavy coquina construction above you. My favorite part was the live demonstration they did with the cannon and period-guns. They do the reinactment a few times a day (on Fridays, Saturdays, and Sundays) with a few park volunteers in super hot, wool costumes, and a park ranger who gives great background information.
Admission is $10 per adult (anyone over 16), but the reciept is good for a few days, so you can come back another day if you missed anything. We were there for about 2 hours total. I would definitely recommend visiting the fort while you're in St. Augustine. It's right next to the beautiful, clear-blue water.
The gift shop inside is where I found out about the passport program and was instantly excited by the idea of it. I bought a collector's edition passport, stamped about 6 stamps in my book from this location alone because they had some fun, extra stamps (usually parks only have one- two for 2016 because it's the centenial year) and then had the dream of visiting all 413 entities of the National Park system.