THEN AND NOW: How New York City's World Trade Center has changed in the 15 years since the 9/11 terrorist attack

Construction on the original towers began in February 1967 with less than a $500 million budget. Architect Minoru Yamasaki designed two 110-story tall towers that were to be constructed as hollow structures supported by steel columns.

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The Port Authority and New Jersey Engineering Department aided in the complex's construction. Over 10,000 workers were involved in the building of the complex, which included both towers and a five-acre outdoor plaza with a 25-foot-tall Fritz Koenig sculpture.

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There were 43,600 windows in the twin towers and over 600,000 square feet of glass. Each building accommodated 50,000 workers and around 200,000 daily visitors.

Upon completion in 1973, the towers stood as the world's tallest buildings at 1,368 feet tall. The Willis Tower, formerly known as the Sears Tower, stole the title in 1974 at 1,451 feet tall.

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The Twin Towers soon became celebrities in themselves. French aerialist Philippe Petit made jaws drop when he tight-roped across the structures on August 7, 1974.

As their popularity increased, the Port Authority created the Windows on the World restaurant on the North Tower's 107 floor. Opened in April 1976, the restaurant delivered aerial city views to diners.

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Architect Yamasaki spoke at the 1973 opening ceremonies. "The World Trade Center is a living symbol of man’s dedication to world peace...beyond the compelling need to make this a monument to world peace, the World Trade Center should, because of its importance, become a representation of man’s belief in humanity, his need for individual dignity, his belief in the cooperation of men, and through this cooperation, his ability to find greatness," he said.

After the September 11 attacks, then-Mayor of New York City Rudy Giuliani proclaimed, "We're going to come out of this stronger than before — politically stronger, economically stronger. The skyline will be made whole again."

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Today, One World Trade Center — or the Freedom Tower, as it's known by many — represents national strength, pride, and America's ability to rebuild.

There was much controversy over One World Trade's design and budget. Daniel Libeskind designed the original master plan in 2003, but it was David M. Childs of Skidmore, Owings & Merrill who eventually designed the complex as it looks today.

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Construction began in 2006 and was completed in 2014. In 2012, The Wall Street Journal reported the building's budget rose to $3.8 billion, making it the world's most expensive new office tower at the time.

The 104-story tower is constructed with more than 2,000 pieces of prismatic glass and stands at 1,776 feet tall. Its height is a subtle tribute to the year America declared its independence.

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The One World Observatory officially opened to the public on May 29, 2015. The observatory sits 1,250 feet above the city, providing visitors with unique views, just as Windows on the World had done nearly four decades before.

The observatory's elevators — called "skypods" — have animated interiors that show New York City's construction throughout history. It only takes 60 seconds to reach the top.

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Visitors are guided through media-walled rooms that discuss the construction of One World Trade before reaching the skypods. Manhattan schist, a type of granite, serves as the building's foundation, and it's 450 million years old.

Today, the site is also home to the National September 11 Memorial and Museum. Names of the nearly 3,000 people who died in the September 11 attacks, in addition to those who died in the 1993 bombing, are inscribed in the bronze panels that border the pools.

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Today, Condé Nast and others call the Freedom Tower home, but we will never forget the twin structures that stood there before.