A feat of Spanish colonial military architecture, the Castillo San Felipe de Barajas is one of Cartagena's most prized attractions. The fortress, whose construction finished in the late 18th century, was never seized – and not for a lack of attempted forays. Part of its sophisticated system of underground tunnels is open to the public; a guided tour (audio guides available) is highly recommended.
Address: Av Arévalo, Cartagena
Opening hours: 8.00-18.00