The Fortress can be visited
Elizabeth Castle

Elizabeth Castle is a fortress located on a tidal island off the coast of Saint Helier, Jersey. Its strategic position and long history make it an example of military architecture.
History+
Construction of Elizabeth Castle began in the late 16th century, when the development of cannons rendered the older Mont Orgueil fortress insufficient for defending the island and the port of Saint Helier. The complex was named after Queen Elizabeth I. In the 17th century, the fortress was first used militarily and played a role in the English Civil War. King Charles II visited the castle multiple times. Later in the same century, a part of the fortress was bombarded by Parliamentarian forces, leading to its surrender.
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This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 49.1761, -2.1254