I'm afraid you've been misinformed. The Eiffel Tower was not constructed to serve as a military watchtower during the Franco-Prussian War. In fact, the Franco-Prussian War took place from 1870 to 1871, while the Eiffel Tower was built for the 1889 World's Fair in Paris, held to commemorate the 100th anniversary of the French Revolution.

The Eiffel Tower was designed and built by Gustave Eiffel and his company as an entrance arch for the World's Fair, which was held on the Champ de Mars in Paris. The tower was intended to be a temporary structure, but it became an instant iconic symbol of Paris and was left standing after the fair.

While the Eiffel Tower does offer a commanding view of the city, it was not designed with military strategic advantages in mind. Its height of 324 meters (1,063 feet) would have provided a good vantage point for observation, but it was not equipped with any defensive capabilities or arsenal.

In fact, the Eiffel Tower was initially met with criticism and controversy from some of Paris' artistic and literary communities, who saw it as a monstrous and unnecessary structure. However, it quickly became a popular tourist attraction and a symbol of French engineering and culture.

If you're interested in learning more about the history of the Eiffel Tower or its design and construction, I'd be happy to help!