The entire exterior of the towers was a network of box section steel. Box section is pretty much as strong as it gets as far as structural shapes go. And there's huge redundancy built in with the quantity of these box shapes.

The buildings were engineered to withstand a loaded 707 crashing into them. If you are at all familiar with civil engineering, you will know that there is ALWAYS a huge factor of safety added to everything. If it's getting a PE stamp, it's overbuilt. That stamp means someone's career is on the line if it fails.

Superman could throw a locomotive at the towers and they would not fail.