You're wrong about the cores, Royston. They were the principal load bearing members. And they were designed and constructed to be able to hold up many times the weight that they actually held up. The perimeter construction served the purpose of holding up the outside edges of the floors, and allowing the building to have some flexibility so it could absorb some of the forces from the wind.

The towers were built with maximum redundancy in mind and the designer of the building said he designed them so that they could withstand several airplane crashes.