Until 1204 the Channel Islands were a part of the Duchy of Normandy. When Normandy was lost to France by King John, Jersey chose to remain loyal to the English Crown. The island was only 14 miles from the coast of France, so the fear of French invasion was a constant concern to the islanders over the centuries during which England and France were at war. The cliffs on the island's north coast provided a natural defence but the bays of the east, west and south coasts were vulnerable to potential invaders.
The Governor of Jersey, General Sir Henry Seymour Conway, decided to build 30 round towers to protect the island's coastline. The inspiration for these towers came from an ancient stone tower in the Bay of Martella in Corsica which held out against a British naval attack. They were round as this shape was regarded as stronger than square ones. However Jersey’s round towers are unique as unlike their English counterparts they have a more elegant design with tall tapering walls. In addition they are generally constructed of local granite and have machicolations (projecting beak like structures high up on the towers and walls) allowing the defenders to protect the base of the tower. The ground floor was used to store ammunition and weapons whilst the upper floor housed up to ten troops and their commanding officer.
You will note that the towers are painted on the seaward side, obviously as an aid to coastal navigation.