Hi Tom.

The towers stood in Manhattan, where tall towers are taken down to build other tall towers frequently. The structure has to be designed with unbuilding in mind, and part of the demolition process can be done with explosives. They are designed to implode, like a big hand comes out of the sky to flatten them.

They stood high enough so that wind was a significant factor for which engineers and architects had to compensate. They wouldn't tip over, especially since they were hit nearer the top. The strike would have needed to be close to the base. Or there would have had to have been a very strong wind, and then it would only (only!) have taken off the upper portion.

An insignificant side note: when my son, my firstborn, was nearly a year old, our little family went to NYC to see the Picasso retrospective at the Museum of Modern Art, before "Guernica", his painting depicting the massacre of a Basque village during the Spanish Civil War, was finally returned to Spain. We celebrated our son's first birthday the night before we went home with dinner at the restaurant at the top of the World Trade Center. The staff loved him: "Nobody ever brings a baby up here! It upsets the suits!" Apart from the lives lost, and the businesses wrecked, it was a very beautiful place.

Mary Anne