Final answer:
Castillo de San Marcos is the name of the fort in San Juan and it was built to defend against imperial challengers.
Step-by-step explanation:
The name of the fort in San Juan is Castillo de San Marcos. It was built by the Spanish to defend against imperial challengers, particularly the English who posed a threat to Spanish Florida and their access to the Caribbean.
Constructed between 1672 and 1695, Castillo de San Marcos is a stone fort that helped Spanish colonists in St. Augustine fend off marauding privateers from rival European countries.