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.

Just an hour's boat trip away from Cartagena lie the magnificent Rosario Islands, which belong to Colombia's only underwater natural park (Rosario and San Bernardo Corals National Natural Park). Abundant marine wildlife (including over 50 species of corals) and some of Colombia's best beaches draw in many in search of relaxation and active pursuits such as snorkelling and diving. Isla Grande is best for kayaking through unique mangrove forests and seeing the phosphorescent plankton light up the waters of a brackish lake at night, while Playa Blanca is Cartagena's most spectacular white-sand beach (also reachable by land). It should be said that the company you choose to tour with has the potential to make or break the experience. Low cost tours can mean multiple intermediate stops en route to your destination, as well as unpleasant surprises such as no transfer back to your hotel. Try and choose a trusted company with consistently good reviews, or opt for a private tour. Mind that seas may be rough at certain times of the year, particularly in December and January, when tours on smaller boats may even become dangerous.

This 18th-century Evangelical church is a neo-Gothic addition to Saarbrücken's diverse architectural scene. The surrounding square is also a major gathering point.