The September 11th Targets, and Their Defense
The targets of the September 11th attacks aside from the four jetliners, were some of the most famous and unique buildings in the world:
These were only the focal points of the attack. The real target of the attack appears to have been something much broader and far-reaching than mere buildings, and even the thousands of innocent victims in the buildings and airplanes. Whatever the motives of the perpetrators of the attack, lower Manhattan, a section of the Pentagon, and four jetliners were the immediate physical targets.
The World Trade Center was a collection of seven buildings in lower Manhattan, including the famous Twin Towers, which were once the world's tallest buildings. By the end of September 11th, 2001, all seven buildings were either leveled or severely damaged and gutted, including Building 7, which was on a separate block.
The Pentagon is the administrative heart of the U.S. military establishment and the largest office building in the world. On September 11th, one of 10 sections was damaged in the attack.
The cities and military installations in the United States are normally defended from airborne attack of any type by NORAD, which monitors all air traffic in North America in real time, and rapidly dispatches interceptors whenever there is a flight emergency. The World Trade Center and the Pentagon would normally be two of the best defended buildings in the world.