Complex sentence: The site was most likely built during the reign of the Inca king Pachacuti Inca Yupanqui and is considered to be the last bastion of resistance and refuge of the Sons of the Sun who fled from the city of Cusco when it was under siege in 1535.
Split into simpler sentences: The site was built during the reign of the Inca king Pachacuti Inca Yupanqui. It may be the last bastion of resistance and refuge of the Sons of the Sun who fled Cusco when it was under siege in 1535.