The Eiffel Tower, although constructed after the Franco-Prussian War for the 1889 World's Fair, can be hypothesized to offer several strategic military advantages if it had existed during the war:

1. **Enhanced Surveillance and Early Warning**: Its height would provide a significant vantage point, allowing surveillance over a wide area, facilitating early detection of enemy movements and enhancing intelligence gathering.

2. **360-Degree Lines of Sight**: The tower's elevation would offer panoramic views, enabling comprehensive monitoring of the surroundings, crucial for detecting threats from any direction.

3. **Improved Communication**: It could serve as a communication hub using signals, flags, or early telegraph systems, enabling faster coordination of military responses.

4. **Defensive Capabilities**: The tower could be fortified as a defensive position, allowing soldiers to engage enemies from a high ground advantage and potentially directing artillery fire more effectively.

5. **Command Center**: It could function as a central command point, enabling officers to oversee battles and direct troops efficiently.

While the Eiffel Tower wasn't used in the Franco-Prussian War, its hypothetical role as a watchtower underscores the importance of elevated structures in military strategy. Additionally, its innovative engineering might have boosted morale and demonstrated technological prowess.