The 300 m iron tower that became the architectural centrepiece for the Exposition Universelle in Paris of 1889 was probably first conceived by Eiffel's young assistant, the Swiss engineer Maurice Koechlin (1856-1946). Without Eiffel's experience, influence and initiative, however, the tower would certainly never have been built. Although based on Eiffel's earlier viaduct pier designs, this tower required much more complicated calculations and construction methods. The vast number of working drawings, prepared under Koechlin's supervision, were extremely precise, making it possible to assemble the prefabricated components on site with virtually no modification. A group of four pivoting cranes was specially built to hoist the components into position. Eiffel's customary attention to questions of wind resistance was reflected in the complex masonry foundations and the hydraulic presses that compensated for shifting in each leg of the tower. Completed on 31 March 1889, the tower, which now bears Eiffel's name, took 26 months to build but never required more than 250 workmen on the site. During the seven months of the exposition, the tower received nearly two million visitors.