It used to be Fort Knox. Nowadays, it's probably the White House. Though, from my understanding, Freedom Tower (i.e., The World Trade Center) is an extremely heavily secured building. It could be the most heavily secured non-government building in the world. After the WTC got bombed in the nineties and then felled in the oughties, the new building its in, Freedom Tower, has major amounts of heavy artillery on its roof protecting it, including antiaircraft guns and antiballistic missiles. And that's just the roof. Throughout the city, there are several other sites also dedicated to its protection with personnel and what is nothing short of material of war. Also, good luck ever driving even a minivan within exploding range of it without preauthorization and a thorough search by security or entering the building with even so much as a Boy Scout knife.