Fort San Marcos which was later known as Fort Marion was a high point of my trip to Saint Augustine from the 7th to the 14th of November of 2006. This fort was once beseiged by the English for over fifty days but was never taken. It is very strong. The walls are at least twenty feet thick. It has a nice museum store with many Florida history books on sale and is a first rate display.
The fort functions as a park in the middle of Saint Augustine. It is a nice place to begin your tour.
I have one picture of the Red Trolley train in this group of pictures. These trolleys go all over Saint Augustine giving guided tours and stopping in all the best locations. I would really recommend spending the twenty dollars for three days access to their services.