On two interconnected islets in St Aubin's Bay, this imposing fortress guards the entrance to the harbour. Originally a 6th-century hermitage, then an abbey, then a 16th-century castle built on the orders of Elizabeth I, the fortress played a key role both during the Civil War and WWII. Visitors can explore the battlements, bunkers, two museums, Upper Ward and Hermitage Rock. At low tide you can walk here along the causeway; otherwise take the amphibious vehicle (£2).
The two engaging museums walk you through the history of the castle, from the murder of the hermit who was subsequently canonised as St Helier and the fortress' role in various major wars, to WWII, when the Nazis heavily fortified it using Eastern European slave labour. The views from the ramparts are spectacular.