They spread the Plague. They also used phycholigical warfare.

They also protected the Silk Road and kept it safe for travellers.

Pope Innocent IV (3):The Great Khan sent a letter back to the pope. He said, “If you should act up to your word then you, the great pope, should come with the monarchs to pay us homage and we should thereupon instruct you.” He went on to inform the pope that he was not subject to the pope, but rather it was the other way around. The Pope's letter was not a success and the Mongols continued to raid.

Pope Innocent IV (2): Pope Innocent sent off a letter with some emissaries to the Great Khan, leader of the Mongols. The Khan at the time was Güyük Khan. He sent some Benedictine monks with the letter. They traveled more than three thousand miles, and it took them more than eighteen months to get to the Mongol camp. And when they finally did get there, they were kept waiting for another few months before they finally had their audience with the Great Khan. The letter was saying that Great Khan should be careful because he is putting himself at the risk of the judgment of God and he wanted peace

Pope Innocent IV: In the 1230s and 1240s, the Mongol armies were raiding Russian towns and cities. These armies even made their way into modern-day Poland and Hungary. They did more raiding than occupying territory—they would come and take what they could and leave a lot of death. It left Eastern Europe and Russia unsettled and it started a panic through the rest of Europe and the Holy Roman Empire. And that is when the pope got involved.

They extended the Grand Canal in China. They also built roads, bridges, and postal stations.

They protected the Silk Road and kept it safe for travellers

They practiced religious tolerance, and patronized the arts.

They destroyed cities, stole from them, and killed millions of people.

Their empire did not last, and they didn't leave behind any architecture.