On the edge of Fidel Castro's Cuba, behind barbed wire and the biggest U.S. minefield in the world, sits a slice of small-town America.

Residents call it "GTMO," pronounced "git-mo," short for U.S. Naval Base, Guantanamo Bay, Cuba - the only American military installation on communist soil.Some 7,000 people live in the seaside community that features six outdoor movie theaters, two schools, one stoplight, dozens of softball fields, an 18-hole golf course, giant warships and a McDonald's.

"I have to worry about a lot of things that few Navy captains have to worry about," says Capt. John Boyd, GTMO's commanding officer and unofficial mayor.

GTMO's chief task is to provide training for the Navy's Atlantic Fleet. But for the past three decades a concern has been that Castro might invade the nearly century-old installation.

Consequently, there is an evacuation drill every three months for non-military personnel. Women and children, with overnight bags, gather on street corners for delivery by bus to ships and planes.

And around-the-clock Marines provide security at a barbed wire and mined 17-mile border that separates the base from the rest of the island.

But all has been relatively quiet since an exchange of gunfire in July 1964, and the biggest complaint isn't threat of attack but eventual boredom with the routine and confinement.

Marine Pfc. Vallard Campbell, 22, of Spiro, Okla., is a member of GTMO's border patrol.

On this sunny day, near the Northeast Gate, Campbell peers through a pair of binoculars at a member of Cuba's "Frontier Brigade" a mile away.

"Initially, its kind of weird looking through the `big eyes' at a communist," says Campbell, speaking over the wind whistling through the Sierra Maestra Mountains.

"But you get use to it - you looking at them and them looking at you. Sometimes they will wave at you."

Does he wave back?

"We aren't suppose to," says Campbell, glancing at a superior officer. "We aren't suppose to make any gestures to them."

Seated at his desk, Lt. Bob Hines, public affairs officer since August 1986, offers his view of "the world of GTMO."

"We go about small-town American life here. You know - getting the kids to school, getting to work on time and doing your job.

"There is something different here from most bases, though. After a year or so, the walls start closing in on you. Some people refer to it as rock fever."

Top gripes include few places to shop (a Navy Exchange and a Marine Exchange), few women (the male-female ratio is 5-to-1) and, despite all the rec-reation facilities and palm tree-lined beaches, a limited number of things to do.

"McDonald's is a godsend," Hines says.

The 45-square mile base in southeast Cuba dates back to a 1903 lease and 1934 treaty that states the United States can stay as long as it wants or until both countries mutually agree it should leave.

For years, the two were good neighbors. Hundreds of Cubans worked at the base, and Marines and sailors went to Havana to drink Cuban rum, smoke Cuban cigars and let off American steam.

But on Jan. 1, 1959, the United States forbade personnel to venture off GTMO and reduced the number of Cuban employees. There are now just 46, all dating back to pre-1959. The action followed Castro's rise to power and the kidnapping and safe return of 29 Marines and sailors.

Shortly after his 1959 revolution, Castro complained, "The Naval base is a dagger plunged into the Cuban soil . . . a base we are not going to take away by force but a piece of land we will never give up."

Tensions soared during the failed 1961 Bay of Pigs invasion by U.S.-backed Cuban exiles and during the 1963 Cuban Missile Crisis that saw President John F. Kennedy pressure the Soviets to remove nuclear weapons from the island.

In February 1964 there was more trouble. In retaliation for the U.S. arrest of 30 Cubans fishing in Florida's water, Castro cut off water to the base. This forced the Navy to install its own desalinization plant.

In July of that same year, Cuba said one of its sentries was fatally shot by a Marine. The United States denied it, saying the Marine fired a warning shot over the sentry's head after the sentry shot at another Marine.

There have been numerous minor "incidents," like the Navy erecting a 50-foot fence near the Northwest Gate to thwart Cubans from throwing rocks at the Marines, and Cubans taunting sentries by shining spotlights in their eyes.

Over the years, some Cubans have reportedly fled to GTMO for asylum. The Navy refuses to talk about it, but Castro responded long ago by planting cactus, known as the "Cactus Curtain," along the border.

Every year, the United States sends Castro a check for $4,085 as rent. He won't even cash them.

That's OK with the Navy brass who intend to keep the strategic foothold in the Caribbean.