Complex sentence: Convicts, slaves, and troops from Cuba were used as labor to erect the structure, which was sited on present-day Rattlesnake Island and has a commanding position over Matanzas Inlet.
Split into simpler sentences: Convicts, slaves, and troops from Cuba were used as labor to erect the structure. The Fort was sited on Rattlesnake Island and has a commanding position over Matanzas Inlet.