Why were watchtowers built along the Great Wall of China and Hadrian's Wall?

Watchtowers and associated fortifications were essential components of the defensive systems for both the Great Wall of China and Hadrian’s Wall, serving primarily as observation points and communication hubs.

Great Wall of China
Watchtowers, often referred to as signal towers or beacons, were strategically placed on hilltops to maximize visibility. Their primary functions included:1
* Military Communication: They utilized smoke signals during the day and fires or lanterns at night to relay information across the frontier.1
* Surveillance and Defense: Towers provided a commanding view of the surrounding terrain, allowing guards to monitor enemy movements and direct troop maneuvers during battles.23
* Logistics: Many towers were self-contained structures that housed soldiers, stored weapons and ammunition, and provided shelter for guards.13

Hadrian’s Wall
Built to guard the northwestern frontier of Roman Britain, the wall incorporated a series of towers and fortlets to manage the border. Their roles included:4
* Frontier Control: The system was designed to control movement across the frontier and counter low-intensity threats from barbarian invaders.4
* Observation: Towers were positioned at regular intervals (every 1/3 Roman mile) to maintain constant surveillance of the territory beyond the wall.4