Final answer:
The fort built around Marietta to protect the citizens was Fort Frederica on St. Simon's Island.
Step-by-step explanation:
The fort that was built around Marietta to protect the citizens was Fort Frederica, located on St. Simon's Island. It was built in 1736 and housed several hundred regular British troops. The fort played a crucial role in defending Georgia against Spanish encroachment during the War of Jenkins' Ear.