Watchtower at Montjuic Castle (Barcelona)

Montjuïc Castle's watchtower wasn't just for spotting enemies; it was crucial for science, as French astronomer Pierre Méchain used it in the 1790s to calculate Barcelona's coordinates, forming the basis for the metric system, and later, its military semaphore telegraph system (with visible crossbars today) connected to other forts, making it a historic communication hub for over 800 years.