There seems to be a misunderstanding in the premise of your question. The Eiffel Tower was **not** primarily built to serve as a military watchtower during the Franco-Prussian War. Here's a brief clarification and then an answer based on the *actual* motivations and capabilities:

### Clarification:
1. **Franco-Prussian War (1870–1871)**: This conflict predated the Eiffel Tower's construction by over a decade. The war ended with France's defeat, and the tower was not conceived until the 1880s.
2. **Actual Purpose**: The Eiffel Tower was built for the 1889 Exposition Universelle (World's Fair) to celebrate the 100th anniversary of the French Revolution. While it had a communications and scientific role, its military utility was secondary and speculative, not its primary intent.

### If the Tower *Had* Been a Military Watchtower (Hypothetical Scenario):
If we entertain the hypothetical idea that the Eiffel Tower was built for military purposes, here are the potential strategic advantages it might have offered the French army in 19th-century Paris:

#### **1. Lines of Sight (Reconnaissance and Surveillance)**
- **Elevation**: At 300 meters (including antennas, it reached 324 meters), the tower would have provided unparalleled visibility over Paris and the surrounding countryside. It could observe troop movements, enemy positions, and fortifications from great distances.
- **360-Degree View**: Unlike ground-level watchtowers, the Eiffel Tower's height would allow for a full, unobstructed panoramic view of the capital, making it ideal for spotting enemy advances or coordinating defensive positions.
- **Signal Transmission**: The tower could have been used to send optical or electrical signals (e.g., using telegraphs or heliographs) to distant outposts, enabling rapid communication of intelligence.

#### **2. Defensive Capabilities**
- **Command Post**: Its height would make it an excellent command center for coordinating artillery fire or directing troops during a siege or battle.
- **Artillery Emplacement**: While the tower itself is not designed to support heavy artillery, smaller defensive guns could potentially be mounted at its base or on its platforms to repel attackers.
- **Deterrence**: The sheer scale of the structure would serve as a psychological deterrent, intimidating enemy forces and signaling France's technological and military prowess.

#### **3. Communication Hub**
- **Telegraphy**: In the 19th century, the tower could have housed telegraph lines, allowing for real-time communication with other military units in and around Paris.
- **Radio (Later)**: Although radio technology was not yet advanced in the 1880s, the tower's height would have been advantageous for early wireless communication experiments, as it was used for radio transmissions by Guglielmo Marconi and others in the early 20th century.

### Conclusion:
While the Eiffel Tower was not built for military purposes, its design would have offered significant strategic advantages if repurposed as a watchtower. Its height, visibility, and potential for communication would have made it a formidable asset for surveillance and defense in 19th-century Paris. However, its actual military use was limited to signaling and radio experiments decades after its construction.