Built in 1973, the twin towers of the World Trade Center were engineered to withstand the force of a 747 slamming into them, the author of a book about the buildings tells Bloomberg.

"The thought was the buildings could take a pretty heavy hit anywhere and survive," says Eric Darton, the author of Divided We Stand: A Biography of New York's World Trade Center.

Darton was in a building in Midtown Manhattan Tuesday morning, and had an unobstructed view of the Trade Center as a second plane crashed into it.

Architect Minoru Yamasaki designed the twin buildings to provide 10 million square feet of office space. The towers consisted of a steel structure with a glass wall exterior. And the 64-metre wide facade of the towers was constructed as a prefabricated steel lattice.

The towers cost $750-million US to build, and stood over 400 metres high -- or 110 stories -- and were once the world's tallest building. They were home to about 500 international companies and 50,000 workers.

Approximately 5,000 tourists visit the World Trade Center on an average day.

The New York city subway system runs under the building and trains from New Jersey terminate at the Trade Center station.