Exo's eye-witness account is very moving; it's still hard to believe what happened that day - even for people like me living a million miles away.
To Chula's point, I'd like to add that some skyscrapers are weaker than others, but lots of them follow this pretty generic floor plan:-
In terms of escape, it's all about how well-built the stair core is. In Britain they have to be conc encased so that they will retain their structural integrity for two hours. Unfortunately, the Twin Towers were built to a less demanding code, and instead of being encased in concrete, the escape core featured steelwork that was protected only by some kind of spray-on intumescent fire-resistant stuff. This wasn't sufficient to protect the steel from sustained heat and proved to be the fatal design weakness, especially when you consider that when hot, steelwork loses strength much faster than even a timber beam, for example, (that takes hours to burn through). Instead of standing for hours, the stair cores collapsed within minutes.
The villains of course are the terrorists, but the terrible death toll is partly due to the authorities who allowed the construction of such tall towers to such an inadequate specification.