Before the end of 1399, Timur started a war with the Mamluk sultan of Egypt Nasir-ad-Din Faraj and invaded Mamluk Syria. Timur's forces took Aleppo[6] He massacred many of the inhabitants, ordering the building of a tower of 20,000 skulls outside the city.[7] After taking Aleppo, Timur continued his advance and besieged Damascus.