The new One World Trade Center is finally open for business, 13 years after the Sept. 11 terror attack. 

The 104-story skyscraper, which cost $3.9 billion to build, dominates the Manhattan skyline and is America's tallest building. It is the centerpiece of the 16-acre site where the twin towers once stood. 

Publishing powerhouse Conde Nast is leaving its Times Square offices and is beginning to move its 3,400 employees into 24 floors of space, becoming the first tenant there. Only 60 percent of the building has been leased so far. 

It has been a long road toward the opening, which was originally scheduled for 2006. Some parts of One World Trade are still being completed, including the highly anticipated observation deck on the 102nd floor.