Did they start building on the site of the World Trade Centre yet?

yes they have begun building on the site..7 world trade is up ..i keep close ear to the matter since i am a 9-11 survivor..they will rebuild the world trade center ..the original had a total of 7 structures..and the rebuilding is supposed to be the same amount..

yes
they make a new building to replace the old trade center

Yes they have. 7 world trade was the first to be rebuilt. They are rebuilding with memorials on the property. The powers that be want them just as high if not higher than before as a sign of never forgetting what transpired and to also inform the terrorists that they have not scared us into not rebuilding the structure because we will overcome as a people.