That's an interesting notion, but the premise contains a significant historical inaccuracy. While the Eiffel Tower *later* proved valuable for military communications, **it absolutely was *not* built as a watchtower during or for the Franco-Prussian War (1870-71)**.

Here's the correction and the actual strategic evolution:

1.  **Timeline Disconnect:**
    *   The Franco-Prussian War ended in 1871.
    *   Construction on the Eiffel Tower began in 1887 and finished in 1889 – **nearly 16 years after the war was over.** It was built as the entrance arch and centerpiece for the 1889 *Exposition Universelle* (World's Fair) celebrating the centennial of the French Revolution.

2.  **The Actual Military Use (WWI):**
    *   The Eiffel Tower's first significant *military* role came decades later, **during World War I (1914-1918)**. Its strategic value had zero to do with being a defensive watchtower against ground troops and everything to do with its height for communications:
        *   **Radio Communications Hub:** The main advantage was height for radio transmission. Rival powers like Germany could intercept radio signals *unless* they were transmitted from a very high point above the ground "hash" and could use directional antennas. The Eiffel Tower provided this critical height advantage over Paris.
        *   **Intercepting Enemy Communications:** French radio operators stationed at the tower achieved significant successes by listening in on German radio traffic. The height improved reception range and clarity.
        *   **Long-Range Communications:** It served as a vital relay point for long-range military radio communications between Paris and the front lines, crucial for command and control.
        *   **Jamming:** It was also used to jam German radio communications, disrupting their command and logistics.
        *   **Signaling:** While outdated by WWI standards for tactical signaling, its height made it a potential secondary site for visual signaling if needed.

**Strategic Advantages Offered (Specifically WWI):**

*   **Communications Height Advantage:** The crowning benefit. At 324 meters (1,063 ft), antennas mounted on the tower provided unparalleled range and signal clarity for both transmission and reception. This broke through ground-level interference and atmospheric limitations.
*   **Central Location in Paris:** Situated in the capital, it was secure within French territory and easily integrated with military HQ infrastructure.
*   **Symbolic and Psychological:** While not tactical, its prominent presence served as a symbol of enduring French resistance during the war.

**Lines of Sight & Defensive Capabilities (The Watchtower Myth Debunked):**

*   **Lines of Sight for Military Observation (Highly Impractical):** While you *could* see incredibly far from the top on a clear day (theoretically over 40 miles), this observation capability was utterly **useless for *defensive* military purposes:**
    *   **Distance:** An attacking army in 1914 would still be many miles away when spotted. The main defensive lines (the trenches) were far too distant for tactical intelligence gathering from a fixed Paris tower.
    *   **Atmospheric Haze & Weather:** Visibility suitable for spotting troop movements over relevant distances was highly unreliable due to fog, smoke, rain, and atmospheric distortion common over the distances involved.
    *   **Target Identification:** Distinguishing troop unit types, numbers, and precise movements from that height with 1910s optics would have been extremely difficult, if not impossible, for any useful tactical purpose. Modern aerial reconnaissance (balloons, then airplanes) rendered this role obsolete anyway, as cameras could get much closer.
    *   **Fixed Location:** Its position was static. An enemy wouldn't need a watchtower to know its location – it was the most famous landmark in Paris. It couldn't be repositioned based on the changing front lines.
    *   **Vulnerability (Defensive Liability):** Far from being a defensive asset, the Tower would have been a giant, **sitting duck** if near a direct confrontation:
        *   It's a massive, un-armored metal structure impossible to camouflage.
        *   It would be an obvious landmark for enemy artillery fire and early aerial bombing (which began in WWI).
        *   Holding it as a defensive position against ground assault would be logistically nightmarish and strategically inferior to more conventional defensive lines.
        *   Sabotage was a constant concern, requiring intense guarding.

**Conclusion:**

While a fascinating and iconic structure, **the Eiffel Tower was never intended or used as a defensive *watchtower*.** Its construction for the 1889 World's Fair occurred long after the Franco-Prussian War had ended. Its true military value emerged unexpectedly during WWI, not for visual observation of troop movements, but as an extremely tall mast providing critical advantages for **radio communications interception, transmission, and jamming** due to its unprecedented height advantage in the heart of Paris. It excelled as a communications relay and intelligence hub, not as a defensive fortress or observation post. The image of soldiers watching for an advancing army from its platforms is a myth.