terrorists dont fight with buildings 
No, they attack obvious, high profile targets in unexpected, vicious, cruel and messy ways. The low-tech way is a car bomb. It's not difficult to mix a batch of home-made explosives, pack it in a van with nails and ballbearings and detonate it in a busy public place.
9/11 was just a bit more high-tech; the concept is sophisticated, but actually carrying it out wasn't difficult. After all, the only moderately tricky bit was to highjack several airliners simultaneously. I suspect that's the only reason that the perpetrators weren't the legitimate pilots of the planes. Don't forget that the other planes didn't make it to their targets, which being military were better defended.
The real damage is not the loss of life or the gap in the New York skyline. It's the backlash within American and, to a lesser extent, the rest of Western society. Look at how pervasive the Homeland Security Agency is; if you'd said 15 years ago that Americans would have willingly accepted it's powers and methods, you would have been laughed at. America was
The Land Of The Free and wasn't prepared to let anyone forget it. Until September, and the entire country was clamouring to be protected.
The terrorists actions made Americans want to live in more of a police state, and by that result alone, they can only consider it a huge success.
The sad thing about this is that nations cannot fight terrorists publicly through bureaucracies. The only effective way is to discretely hunt the murderous oppsers down and make them disappear for ever. And that's just as scary a thing for a civilised democratic society to do.