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Battlefield communications before the advent of strict lines of communication were difficult. Communication was done through musical signals (drums and horns), audible commands, mounted messengers, and visual signals such as flags.
Fire beacons were used in many places where there was a network of towers or castles visible one from another. On the border between England and scotland a line of Peel Towers was built for exactly this purpose. In Scandinavia many hill forts were part of beacon networks to warn against invading pillagers. In Wales, the Brecon Beacons were named for beacons used to warn of approaching English raiders. In England, the most famous examples are the beacons used in Elizabethan England to warn of the approaching Spanish Armada. Many hills in England were named Beacon Hill after such beacons. In the Languedoc, where castles were often built on mountain tops, castles were almost invariably in view of at least one one other castle.
Carrier pigeons historically carried messages only one way, to their home. They had to be transported manually before another flight.
By placing their food at one location and their home at another location, pigeons have been trained to fly back and forth up to twice a day reliably. This setup allows Pigeons to cover 160 km round trip.