Complex sentence: Castle Cornet is a large island castle in Guernsey, and former tidal island, also known as Cornet Rock or Castle Rock, its importance was as a guardian not only of the Island, but of the roadstead.
Split into simpler sentences: Castle Cornet is a large island castle in Guernsey, and former tidal island, also known as Cornet Rock or Castle Rock. Its importance was as a guardian not only of the Island, but of the roadstead.