Castillo de San Marcos

Beginning in 1738, under the supervision of Spanish engineer Pedro Ruiz de Olano, the interior of the fort was redesigned and rebuilt. Interior rooms were made deeper, and vaulted ceilings replaced the original wooden ones. The vaulted ceilings allowed for better protection from bombardments and allowed for cannon to be placed along the gun deck, not just at the corner bastions. The new ceilings required the height of the exterior wall to be increased from 26 to 33 feet (10 m).

In 1670, Charles Town (modern-day Charleston, South Carolina) was founded by English colonists. As it was just two days' sail from St. Augustine, the English settlement and encroachment of English traders into Spanish territory spurred the Spanish in their construction of a fort. Slaves from the Carolina colony began escaping to St Augustine in 1687, where the Spanish agreed to free (and employ) them if they converted to Catholicism. When an English major from Carolina attempted to retrieve escapees in 1688, the Spanish Governor Diego de Quiroga refused.

In 1702, English colonial forces under the command of Carolina Governor James Moore Sr. embarked on an expedition to capture St. Augustine early in Queen Anne's War. The English laid siege to St. Augustine in November 1702. About 1,500 town residents and soldiers were crammed into the fort during the two-month siege. The small English cannons had little effect on the walls of the fort, because the coquina masonry was very effective at absorbing the impact of cannonballs causing them to sink into the walls, rather than shattering or puncturing them.