The Eiffel Tower, built by Gustave Eiffel in 1889. Eiffel had a permit for the tower to stand for 20 years, meaning it would have had to be dismantled in 1909, when its ownership would revert to the City of Paris. The City had planned to tear it down but as the tower proved valuable for communication purposes, it was allowed to remain after the expiry of the permit. The military used it to dispatch Parisian taxis to the front line of the Marne.
(Thx Wikipedia bc the 3 blondies who gave me a tour in Paris did not not the dates and so one....poor them!)