Complex sentence: Construction of the tower began in 1813 on a rocky cliff during the War of 1812 to guard the land approaches to the city against an American overland invasion; however, the tower was not completed until 1815, after the war had ended.
Split into simpler sentences: Construction of the tower began in 1813, during the War of 1812, on a rocky cliff to guard the land approaches to the city. It was intended to defend against an overland invasion from the United States, but the tower was not completed until 1815, after the war had ended.