The Torre del Saraceno is located along the east coast of the Isola del Giglio in the heart of Giglio Porto. The tower originally belonged to the Abbey of Tre Fontane in Rome in the Middle Ages, before falling into the hands of the Aldobrandeschi, who seized control of the entire island in the 12th century. In the centuries after the Aldobrandeschi, the tower was subjected to a number of pirate attacks - many of which severely damaged the structure, despite its heavy outer barbican and other defensive elements.
In a random aside, the Torre del Saraceno actually gets its name from the pirate fleet of the Saracens, who inflicted some of the worst damage on the tower during one of their violent raids. When Italy was unified, the tower was abandoned and today is a little worse for wear.
You still can walk around it and admire the stone blocks, terrace and parapet, but you can't go inside. Along the outer walls are different quadrangular windows used for watching, firing and holding gunboats.
The tower itself is also a great place to admire the sea and surrounding bay, especially during the day when boats are sailing to and from the island.