Frustrated, sweating tourists with hands on their hips stood in front of a seaside 16th-century fortress on Tuesday after discovering it was one of several sites in the U.S. territory of Puerto Rico closed by the partial federal government shutdown.

The majority of Puerto Rico’s 10,000 federal employees were affected by the shutdown, which led to closure of the imposing San Felipe del Morro fort in San Juan, along with other tourist destinations including El Yunque, the only tropical rain forest in the U.S. National Forest system. Also affected are national wildlife refuges in the nearby islands of Culebra and Vieques, according to Resident Commissioner Pedro Pierluisi.

“The shutdown is denying public access to every one of Puerto Rico’s federally staffed and protected natural treasures,” he said. “Closure of these sites, particularly El Yunque, harms Puerto Rico’s tourism base and hurts small businesses that depend on tourists as their customers.”

Several visitors learned about the closure of the San Felipe del Morro fort only after trudging along the long entrance path in the sticky tropical heat.

“I walked all the way up there and almost died,” said Jim Kuehn, a 68-year-old from Pine Mountain, Georgia, who was visiting Puerto Rico with his son. He noted that no one had yet posted any closure notices.

Jasmine Jaw, a frustrated 27-year-old from San Jose, California, had already bought tickets. She knocked on the massive wooden double doors Tuesday morning and tried peering in when she got no answer.

“We wanted to see this,” she said. “We’ve come all this way.”

“It’s definitely disappointing,” said Kaustubh Girme, a 30-year-old from Boston who was visiting Puerto Rico with his wife. “It’s really frustrating that ordinary people have to suffer for something they couldn’t do.”

U.S. Forest Service Agent Carmelo Ortiz said he turned away numerous tourists trying to visit El Yunque on Tuesday.

The National Guard of Puerto Rico said more than 600 of its 800-strong force are not working, along with more than 100 federal government contractors.

In the neighboring U.S. Virgin Islands, nearly 700 federal employees who work there have been affected, according to Congressional Delegate Donna Christensen.

Among those still on the job were some 180 air traffic controllers, more than 600 U.S. Customs and Border Protection agents and employees at existing Head Start centers in Puerto Rico.

Photo Credit: Alicia Houtrouw of California turns and leaves the entrance of the San Felipe del Morro castle after being told by park rangers the castle was closed due to the U.S. government shutdown, in San Juan, Puerto Rico, Tuesday, Oct. 1, 2013. Ricardo Arduengo / Associated Press