Final answer:
The description suggests the country in question is Puerto Rico, which is known for its ancient streets, historic forts, national forests, indigenous history, and the popularity of flan.
Step-by-step explanation:
Based on the clues provided, the country being described with ancient streets, structures, a history of the Spanish and other indigenous groups, a tour of a fort, exploration of national forests and exotic caverns, and the popular dessert flan, it seems the country in question is Puerto Rico. The mention of the Castillo de San Marcos, which is a fortification in St. Augustine, Florida, is a hint towards Spanish colonization, which heavily influenced Puerto Rico.
Furthermore, references to the indigenous population and the Spanish conquest align with the history of Puerto Rico. The inclusion of the National Park Service's multimedia resources on the Castillo de San Marcos fort directly points to the region which was under Spanish influence and is closely connected with Puerto Rico's history.