Correction: Earlier versions of this article referred to the Navy Yard as being in Southwest Washington. It is in Southeast.

Behind the imposing red-brick wall, the Washington Navy Yard is a self-contained village within the capital city, an enclave of high-tech warfare, military history and harried office workers along the Anacostia River.

From streetside, the wall offers only cryptic glimpses of period rooftops and modern office blocks. Even peeking through the Latrobe Gate on M Street SE (almost no one uses this grand entry except the chief of naval operations) suggests little of the 65 tightly packed acres, which contain restaurants, stores, a barbershop, a bank, two gyms, a karaoke bar for Friday night and a chapel for Sunday morning. Until recently, the denizens of the Navy Yard needed to have little to do with the sometimes seedy urban blocks outside.

“It’s a different world. You’re leaving D.C. and entering the Navy Yard,” said Colin Galle, who recently ended a stint in an office adjacent to the parade ground. “A lot of times I would come in in the morning and not leave again until the end of the day.”

But in recent years, the wall has begun to crack. As the old cement plants and strip bars along the Southeast waterfront have given way to brewpubs, condominiums and a baseball stadium, Navy workers have ventured more and more into the streets outside. The parade of Metro riders coming from the Navy Yard station no longer marches unbroken down M Street but scatters through the play spaces and fountains of Yards Park between the base and the stadium.

Slowly, the base has become less of a fortress within the neighborhood — a shift that neighbors hope is not reversed by the shooting there Monday that left 12 victims dead. A new public bike path passes through the sliver of the compound closest to the river. This summer, officials experimented with opening some of the riverside gates to visitors eager to see the vast National Museum of the U.S. Navy within.

Read eyewitness accounts from the Navy Yard shooting.

“When the neighborhood was less developed, and there were fewer places to go, it had that reputation of being a walled-off compound. That’s definitely changed,” said David Garber, the area’s advisory neighborhood commissioner. “They opened up a lot; people are filtering in and out in both directions.”

The day starts early at the Navy Yard, thanks to both the military clock and the need to outflank D.C. traffic. The cars start lining up at the Sixth, Ninth and O street gates by 6 a.m. Those arriving later than 8 a.m. often have to pay $10 at the public lots across M Street — although that crunch has eased a bit since budget-mandated furloughs hit in July.

“It’s gotten a little easier to find parking on Mondays and Fridays,” said Lt. Cmdr. Heidi Lenzini of the Naval History and Heritage Command.

For the history-minded, such as Lenzini, entering the country’s oldest Navy base each day means basking a bit in the unbroken mission of the place. Since the age of sail, these same blocks and some of the same buildings have existed to protect the republic’s interest on the high seas.

“This is where so much of our history happened; you’re just surrounded by it,” Lenzini said. “This is where the first Congresses complained about the cost of the first clipper ships. Lincoln came here to see the Monitor during the Civil War. For me it’s like 238 years of Navy history — go!”

Others rarely wander into those older precincts, where the edifices missed by British torches still stand (among them Tingey House and the main gate designed by U.S. Capitol architect Benjamin Latrobe). They go straight from car to cubicle, in buildings that are crammed corner to corner in a footprint that has steadily shrunk to what it is today: the Naval Sea Systems Command, the Military Sealift Command, the Office of the Naval Inspector General, the Navy Judge Advocate General’s Corps (where three former U.S. Naval Academy football players faced rape charges a few weeks ago).

“This is prime real estate for the Navy, being so close to Capitol Hill,” said Galle, a graduate student at Stanford University. “They squeeze as many agencies as they can in there.”

Many of the structures have been repurposed from an earlier age of warfare. Building 197, where the gunman unleashed his deadly assault Monday, was once a shipbuilding facility, according to Patrick McNeil, a contractor who works on the fourth floor.

“They’ve kept some of the old cranes and some of the superstructure in the atrium,” McNeil said. There are also models of vessels old and new on display in the space that has now become infamous as the scene of mass murder. “You see the historic aspect of these buildings every day.”

When employees do take a break, there are precious few open spaces for them relax. McNeil sometimes goes down to the patches of green amid the ancient cannons near the river. Galle will grab coffee at some outdoor tables close to the 11th Street Bridge, near where the admiral’s barge belonging to the Navy’s highest officer is moored.

Sometimes there are officer retirement ceremonies open to office workers, with multi-gun salutes and occasionally hors d’oeuvres. While schoolkids visiting the Navy Museum sometimes tramp the ceremonial courtyard, it is not a play space.

“It’s a parade ground,” said Galle. “You’re not out there throwing the Frisbee.”

But he, like hundreds of Navy Yard workers, finds himself going outside the gates for a break from the demands of national security. The Navy Yard was established in 1799 and eventually stretched all the way to First Street SE. Much of that surrounding land is being transformed in one of the city’s largest mixed-use real estate projects, the Yards. The $2 billion, 42-acre project is putting the bones of the old industrial Navy buildings to new use. A boilermaker shop has become a retail and dining pavilion, while a pattern and joiner shop has been turned into luxury waterfront apartments.

Construction of a Harris Teeter grocery store and more apartments is underway, and the developer is planning a movie theater.

“It’s pulled people out of the Navy Yard more than ever,” Garber said. “It would be really unfortunate if it were to become less open to the city again because of what happened Monday.”

Jonathan O’Connell contributed to this report.