Is it a Fort or Castle?
A very interesting question, and there really isn't a 100% clear answer. According to Wikipedia, "the definition of a castle accepted amongst academics is 'a private fortified residence'." Merriam-Webster dictionary says a castle is "a large fortified building or set of buildings; a retreat safe against intrusion or invasion." Oxford English Dictionary says it is "a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and in many cases a moat."
The Castillo de San Marcos fits all of those definitions in one way, or at one time, or another. When it was first built, the governor of St. Augustine resided inside the building, which would make it a "private fortified residence." It is most definitely a large, fortified building that provided a retreat safe against invasion for the people of St. Augustine (at one time, it housed over 1,500 people for 51 days while the English laid siege!). And it also has thick walls, battlements, towers, and a moat.
Most of the fortifications the Spanish built in the New World were named Castillos. Perhaps it is a hold-over from medieval times, meant to inspire their people and instill fear in their enemies? The Spanish were fond of decoration and embellishment in the physical designs of their fortresses; they may have felt the same about their names. As of yet, there has been no documentation found explaining exactly why they chose to use Castillo rather than Fuerte or Fortaleza. However, it is interesting to note that the wooden fort that preceded the current stone one was also
called Castillo de San Marcos.
Also interesting is the fact that it does later become referred to as a fort. When the British gained Florida through the 1763 Treaty of Paris, they renamed the building Fort Saint Mark, and the United States Army decided in 1825 to call it Fort Marion. Under those occupations, it was indeed used for purely military function. The British and the Americans did not plan to use it as a place of refuge for the citizens of St. Augustine; they both used as barracks, for military storage, and a few times as a military prison. The National Park Service and United States Congress decided to restore its original name in 1942, in honor of its unique Spanish history, so it went back to Castillo de San Marcos for good.