Fort Saint-Jean guards the northern side of the entrance to the Vieux-Port. Louis XIV ordered its construction in 1660, supposedly to defend the city from foreign attack — though the cannons in fact pointed inland, after a recent revolt by Marseille against royal authority. The fort kept that role through the Revolution, the Empire and into the 20th century before becoming a museum in the 2000s.
The buildings are now part of the MuCEM complex. A pedestrian bridge connects the fort to the Mucem's J4 building over the harbour mouth, and another spans to the Esplanade de la Tourette near the Cathédrale de la Major. You can enter the fort grounds free of charge (the museum exhibits inside require a Mucem ticket) and wander the ramparts for some of the best views in central Marseille: the Vieux-Port behind you, the Frioul islands and Château d'If straight out to sea, the cathedral and the Panier to the north.