There seems to be a misunderstanding in your question. The Eiffel Tower was not constructed to serve as a military watchtower during the Franco-Prussian War. The Franco-Prussian War took place from 1870 to 1871, whereas the construction of the Eiffel Tower began in 1887 and was completed in 1889. The primary motivation for the construction of the Eiffel Tower was to celebrate the centennial of the French Revolution and to demonstrate France's industrial prowess to the world.

The Eiffel Tower was designed as the centerpiece of the 1889 Exposition Universelle (World's Fair) in Paris. Its designer, Gustave Eiffel, and his team created it to showcase the potential of wrought iron for architectural purposes. While it's true that the tower later served some military and communication purposes, such as radio transmissions during World War I, it was not initially conceived with military strategy in mind.

The Eiffel Tower's height (324 meters at the time of its construction, which was the tallest structure in the world until 1930) theoretically could have offered advantageous lines of sight for surveillance and communication purposes, but that was not its primary intended use. Its construction and the era's technology did not align with it being an effective military watchtower during its inception.