Complex sentence: Gibraltar lived in relative peace for over 200 years after the tenth siege, during which time both the "Reconquista" and the unification of Spain had been completed and thus the Rock's importance as a fortress diminished and its defences were neglected.
Split into simpler sentences: Gibraltar lived in relative peace for over 200 years after the tenth siege, during which time both the "Reconquista" and the unification of Spain were completed. The Rock's importance as a fortress diminished and its defences were neglected.