History.com: The World Trade Center, by the Numbers

When the World Trade Center's Twin Towers opened to the public in 1973, they were the tallest buildings in the world. Statistics: They rose a quarter-mile in the sky, contained 15 miles of elevator shafts, and nearly 44,000 windows. From the South Tower observation deck on a clear day, visitors could see 45 miles. The Trade Center complex was so big, it had its own zip code. But some of the same impressive architectural elements may have also helped worsen the tragedy on the fateful morning of September 11, 2001.