Dotted along Sardinia’s 1,800 kilometers of coastline is a network of over 100 historical watchtowers, built mostly during Spanish rule in the 16th century. Among the most strategic is the Tower of San Giovanni, placed on an elevated lookout to spot incoming Ottoman and Barbary pirate raids. The towers were built within line of sight of one another: upon spotting enemy ships, guards transmitted smoke signals by day and beacon fires by night. This optical warning network could alert inland settlements and military garrisons in a matter of minutes.