The Il-Khanate period provided several innovations to dome-building that eventually enabled the Persians to construct much taller structures. These changes later paved the way for Safavid architecture. The pinnacle of Il-Khanate architecture was reached with the construction of the Soltaniyeh Dome (1302–1312) in Zanjan, Iran, which measures 50 m in height and 25 m in diameter, making it the 3rd largest and the tallest masonry dome ever erected. The thin, double-shelled dome was reinforced by arches between the layers. The tomb of Öljeitü in Soltaniyeh is one of the greatest and most impressive monuments in Iran, despite many later depredations.