Nearly 3,000 people were killed, including men, women and children.
The Americans at home watched in terror as two planes, hijacked by terrorists, crashed into the World Trade Center towers, forcing some inside to fall or leap to their deaths.
Completed in 1973 as the tallest buildings in New York City, the South Tower rose to 1,730 feet height with a large antenna and the North Tower at 1,362 feet high.
Each massive tower weighed more than 250,000 tons and contained 110 stories.
Inside there were 99 elevators, 21,800 windows and each floor was one acre in size.
The North Tower and South Tower provided nearly 10 million square feet of office space for 35,000 people and 430 companies.
The World Trade Center complex had even its own zip code, 10048, and the Twin Towers were surrounded by five more buildings including a police desk in 5 WTC.
The towers attracted roughly 70,000 tourists and commuters every day.
Before 9/11, the Twin Towers were a symbol of the city.
But the city fell however when the towers fell, to whom it fell.
However, life in the Big Apple could not stop and just hours after the attack, then-Mayor Rudy Giuliani promised to rebuild.
Several months later, the newly appointed Mayor Michael Bloomberg pledged that he would dedicate himself to building a better New York City. Giuliani encouraged buildings to build low, according to Quartz and U.S. Model Building and Fire Codes were revised pending an investigation.
In a recent op-ed the Wall Street Journal's Michael Lewis notes the significance of the 2007 FEMA 'Site and Urban Design for Security: Guidance Against Potential Terrorist Attacks, which promoted three layers of defense strategy that are similar to those surrounding medieval castles.
At the same time, many of the actions advocated by FEMA compel buildings users to remain invisible.
Security has been woven into urban development, especially in New York City.
And yet, Lewis adds, 20 years later, skyscrapers and supertall buildings that fill city skies to!
The Council on Tall Buildings and Urban Habitat reports that 84% of the world's skyscrapers have been built over 200 meters over the last two decades, though most are not in the U.S.
The reason, according to many experts in the field, is in increased building science and improved confidence over the past 20 years.
There is a direct relationship between developments in building science related to the construction of supertall towers and the perception of enhanced safety which has allowed commercially viable towers in New York City, said Carl Galioto, president of the global design and architecture firm HOK.