How did they build the Tower so quickly?

On June 12, 1886, the decision was made to build the 984-foot (300-meter) tower proposed by Gustave Eiffel during the competition for the 1889 Exposition Universelle, which was scheduled to open on May 15, 1889. That left him only three years to build the tower. But six months were still needed for gaining the concession of the land from the City of Paris, for the negotiations with the State and for the financial package personally assumed by Eiffel himself.

Work began on January 26th, 1887, with the construction of sixteen masonry foundation blocks, one per edge. Two of the tower's pillars sink below the level of the Seine and required the use of watertight caissons with compressed air. The foundations were completed six months later and the assembly of the metal structure began on July 1. The secret of this quick assembly was the complete prefabrication of the 12,000 parts of the tower in Eiffel's workshops in Levallois-Perret, which had already begun during the construction of the foundations. There, all parts were calculated, drawn, cut, drilled, pre-assembled with rivets, then sent to the site and sent back to the workshop if they were defective.

Two-thirds of the approximately 2,500,000 rivets in the tower were thereby inserted at the factory. Modest steam cranes and between 150 and 300 well-supervised workers were enough to assemble all the metal parts thus prefabricated in 22 months on the Champ de Mars. A masterfully executed project!