The original engineer who worked on the WTC was understandably upset. The towers had been designed to withstand the impact of a 707 airliner--the largest plane at the time. The burning jet fuel--a source of ignition far beyond the type of fire that would normally be expected in a skyscraper--was the downfall of the design, weakening the steel structures that clamped the floors to the core, and letting the floors pancake down. A rebuilt tower could be beefed up even more, but I am not certain there is a practical way to make the structure immune to continued fire.

Weapons to attack approaching planes would be almost useless. The terrorists were flying civilian airliners that only turned toward their targets minutes before impact. What good would a weapon do? Shoot down an airliner on a beeline for you, and it would simply drop its burning wreckage on some other portion of Manhattan. And what if it never intended to hit you? How do you know for sure until it's too late? A wrong guess and you drop an airliner full of innocent people into the city.

Face it, there isn't any way to protect yourself completely from such attacks. The best you can do is have an active intelligence network to prevent such attacks. People have been ignoring many acts of terrorism that did NOT occur because of the FBI and others nipping them in the bud. As just one example of this, Shaykh Rahman was convicted in 1995 for plotting to bomb several New York City landmarks, including the United Nations building and the Holland and Lincoln tunnels.