- **Quick correction**: The Eiffel Tower was not conceived for the Franco‑Prussian War (1870–71). It was designed for the 1889 Exposition Universelle. Its real military value emerged later, primarily as a wireless (radio) station in the early 1900s and during World War I.

- **What it actually offered (late 19th–early 20th century)**:
  - **Long‑range communications**: Its height made it an exceptional radio mast. French signals officers (notably Gustave Ferrié) used it for wireless telegraphy, enabling resilient command-and-control, message relay, and later interception/jamming of enemy signals.
  - **Intelligence/early warning**: Elevated antennas improved reception and direction‑finding; in WWI, it aided interception that supported operations such as the Marne.
  - **Meteorology and navigation**: Height favored meteorological observations and later aviation signaling.

- **If we consider a hypothetical 1870–71 “watchtower” role in Paris**:
  - **Lines of sight**:
    - From ~300 m elevation, geometric horizon ≈ 3.57√h km ≈ 62 km (often 70+ km with atmospheric refraction). The observation platforms (~276 m) still yield ≈ 59 km.
    - This easily covers all of Paris, the Thiers wall, and the ring of detached forts (e.g., Mont‑Valérien, Issy, Ivry, Aubervilliers), which sit roughly 5–15 km from the center—well within continuous visual range.
    - Clear sight lines between the tower and all peripheral forts would enable rapid optical signaling (semaphore/heliograph) redundancy alongside telegraph lines.
  - **Defensive utility**:
    - **Early warning**: Rapid spotting of troop movements, artillery emplacements, and columns approaching across the relatively flat Île‑de‑France terrain.
    - **Artillery fire correction**: Centralized observation to adjust counter‑battery fire and coordinate sectors across multiple forts.
    - **Signal relay hub**: A single, dominating node to fan out orders/alerts to all forts and city command posts, even if ground lines were cut.
  - **Limitations and risks**:
    - **Weather/smoke**: Fog, rain, winter light, and battlefield smoke severely degrade optical observation in 19th‑century conditions.
    - **Targetability**: A tall, conspicuous structure is an obvious aiming point; although difficult to destroy with 1870s artillery at range, it could be damaged and its observers endangered.
    - **No intrinsic “defense”**: It offers awareness and coordination, not protection; it isn’t a fort and provides no direct firepower or meaningful cover.

In short, while not built for the Franco‑Prussian War, the tower’s later real-world advantages were communications and intelligence. Hypothetically, had it existed in 1870–71, its commanding lines of sight over all Paris and the outer forts would have made it a powerful observation and signaling hub, with effectiveness constrained by weather and its vulnerability as a landmark.