During the Middle Ages, defense in Sardinia relied on a network of inland castles and fortresses, such as the Malaspina Castle in Bosa. These fortresses were built on elevated outcrops to maintain a direct line of sight with coastal watchtowers. In times of danger, guards sent smoke signals by day and fires by night. This visual communication system could relay warnings from coastal towers to inland valleys in minutes, showing how the island’s mountainous terrain was repurposed as a military asset.