Egypt had a powerful empire under Pharaoh Ramses II in the 13th century BC. However, after Ramses' death invasions occurred which caused the empire to decline. Lower Nubia was conquered by Egypt between 1975-1640 BC and all of Nubia was conquered by 1500 BC. Over the centuries Egypt was invaded and conquered by various powers including the Nubians, Persians, Greeks, and Ottoman Turks. The sail and calendar were invented in ancient Egypt which helped development.