Real Instituto y Observatorio de la Armada Monument · San Fernando, Bahía de Cádiz, Cádiz A scientific research and cultural outreach centre belonging to the Spanish Navy, considered the oldest modern observatory in Spain and one of the most important in the world. It functions as an astronomical and geophysical observatory and as a higher education centre for the scientific personnel of the Navy, and holds a library of 30,000 volumes. The building, one of the finest examples of neoclassical architecture in the Cádiz area, was built at the end of the 18th century on the Torre Alta hill, with great simplicity and purity of line, replacing the earlier headquarters located in Cádiz, whose altitude was insufficient for astronomical observation. Photo: Sobreira (CC BY-SA 4.0) What to see and do in San Fernando