I have gone through three version of the World Trade Center. This is Version 1, from April 2000.
The exterior contained over 12,500 pieces, or about 500 pieces per floor! (The walls were 3 studs thick and contained mostly small white 1x1 and 1x2 pieces.) The building was 6 feet 6 inches tall, including the 1 foot antannae.
The tower was hinged on its center, and opened to reveal the inside. It was constructed on two 16x32 baseplates. There were 25 floors, including a 3-story lobby, a 2-story sky-lobby, and an observation deck on the roof. The whole building would come apart in stackable "chunks", which made moving it around and swinging it open less insane.
When complete, each tower was to be 5.5 feet tall. Tower 1 had a 1-foot tall antennae on top, and tower 2 was to have an observation deck and helipad. Buildings 3 through 6 would each be just under 1 foot tall. The whole center was to be situated on an 11-square-foot open air plaza. Underneath, there was to be a shopping plaza and subway lines. This is a sketch I did when I was planning everything back in late 1999 / early 2000.
About 25% of the building was populated before it was knocked over. I had created a 3-story tall lobby which contained a waiting area, a reception desk, and 4 elevators. There also was a photo studio, building security, a dentist, a corporate office, and a hallway with a Coke vending machine.
Actually, sticking together all those little 1x1 and 1x2 pieces would get VERY tedious, and after a while they would really hurt my thumbs. Each floor had over 500 little 1x1 and 1x2 pieces.
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From 2001 to 2003, I lived a few blocks from the World Trade Center site. I passed ground zero every day. Inspired by the gaping void in my neighborhood's skyline, I was inspired to begin re-building the World Trade Center. This is a prototype for a model that has more detail, and could be built entirely out of pieces available in bulk from LEGO Shop At Home.
It proved to be rather expensive, however, and I never completed the new model.