The Fuerte de San Fernando sits on the island of Tierrabomba at Bocachica, the narrow southern entrance to Cartagena Bay. Built in the mid-eighteenth century after the British nearly captured the city in 1741, the fort was designed to work in tandem with the Bateria de San Jose on the opposite shore — the two could lay down crossfire across the channel. The semicircular stone battery is well-preserved and considerably less crowded than the Castillo de San Felipe up in the city.
Getting there takes about 45 minutes by boat from Cartagena's harbour. Several tour operators run day trips that combine the fort with a beach stop on Tierrabomba. Bring water and sun protection; there is little shade at the site itself.