I need to correct a fundamental misconception in your question. The Eiffel Tower was not built as a military watchtower during the Franco-Prussian War, nor was that its primary motivation.

The actual facts are:
- The Franco-Prussian War occurred in 1870-1871
- The Eiffel Tower was constructed from 1887-1889
- It was built specifically as the centerpiece for the 1889 Exposition Universelle (World's Fair) in Paris, commemorating the centennial of the French Revolution

The tower was designed by Gustave Eiffel as a demonstration of French engineering prowess and modern industrial capabilities. It was meant to be a temporary structure showcasing what could be achieved with iron construction techniques.

While the Eiffel Tower did later serve some military purposes (particularly as a radio transmission tower during World War I, which helped intercept enemy communications), this was not why it was built. The tower's height did make it useful for scientific experiments and telecommunications, which helped justify keeping it standing after the exposition, when it was originally scheduled to be demolished.

The strategic value of high observation points in 19th-century Paris was indeed recognized, but this was addressed through other structures and fortifications, not the Eiffel Tower, which came nearly two decades after the Franco-Prussian War ended.