New York City, often called the City of New York, to distinguish it from the State of New York, of which it is a part—is the largest city and largest metropolitan area, by population, in the United States, and also one of the ten primary cities in the crossover series. It is an international center for politics, theater, music, fashion, finance, and culture. New York City is classified as a global city, and houses the United Nations. The city is home to many landmarks including the Statue of Liberty, Empire State Building, Flatiron Building, Chrysler Building, and Saint Patrick's Cathedral. The World Trade Center, also known as the Twin Towers, adorned the skyline until a terrorist attack on September 11th, 2001.