French engineer Gutave Eiffel originally planned to build the tower in Barcelona, for the Universal Exposition of 1888, but the Consistory of the town refused the project. Eiffel submitted his draft to those responsible for the next Universal Exposition, which was to be held in Paris: a permit for the tower to stand was accepted for 20 years. The structure was built as the entrance arch for the 1889 Exposition Universelle of Paris, marking the centennial celebration of the French Revolution. Three hundred workers joined together 18,038 pieces of puddled iron (a very pure form of structural iron), using two and a half million rivets, in a structural design by Maurice Koechlin. The Eiffel Tower is now the most visited paid monument in the world.