One World Trade Center’s official opening marked a symbolic return to some sense of normalcy for the site where the towers toppled more than 13 years ago.

“The New York City skyline is whole again,” said Patrick Foye, executive director of the Port Authority of New York and New Jersey, which owns both the building and the 16-acre World Trade Center site.

Steps away from the new tower are two memorial fountains built on the footprints of the decimated twin towers, a reminder of the more than 2,700 people who died in the terrorist attack.

Conde Nast, publisher of Vogue, The New Yorker and Vanity Fair, is expected to move in about 3,000 more employees by early next year, eventually occupying 25 floors of the $3.9 billion, 104-story tower, the nation’s tallest building.

Amid Monday’s celebratory tour of parts of 1 World Trade Center, Conde Nast officials declined to comment on employees’ possible fears about working in the new building.

Foye countered that it’s “the most secure office building in America.” And its chief architect, T.J. Gottesdiener, said the high-rise was built with steel-reinforced concrete that makes it as terror attack-proof as possible — much stronger than the original towers that collapsed on themselves when the hijacked planes hit.

The stairwells are built with a hardened concrete core; and they’re wider to allow firefighters to move while people exit. The building’s mechanical systems are also encased in hardened concrete.

“If my son told me he had a job in the trade center Tower 1, I would have no qualms about him being there,” Gottesdiener said.

After 9/11, he said, architects took pains to figure out new ways to make a high-rise safer, working with the New York Fire Department, buildings officials and police, while learning from new techniques from construction in cities worldwide.

Finally, computerized simulations were used to calculate what would happen with people in the building.

One World Trade Center is 60 percent leased, with an additional 80,000 square feet going to advertising firm KiDS Creative, stadium operator Legends Hospitality, BMB Group investment adviser, and Servcorp, a provider of executive offices.

The government’s General Services Administration signed up for 275,000 square feet, and the China Center, a trade and cultural facility, will cover 191,000 square feet.

The eight-year construction of the skyscraper came after years of political, financial and legal infighting that threatened to derail the project.

The bickering slowly died down as two other towers started going up on the southeast end of the site: the now completed 4 World Trade Center, whose anchor tenant is the Port Authority, which started moving in last week; and 3 World Trade Center, which is slowly rising.

Still, it’s a bittersweet victory, one achieved while haunted by the past.

“The city and the world were watching us, and we had to do it right, to do it better than before,” Gottesdiener said. “And we did it, we finally did it.”