What problems did the mongols face in holding their vast empire together?

How does literature reflect aspects of European culture in the Middle-Ages? What did Charlemagne do to try to unify western Europe after winning the war against the Saxons? Who built the aqueduct of Segovia? What city was attacked by crusades in the fourth crusades? What did WRVS stand for? What did a cordwainer do in the middle ages? Why was free grain given to the poor by Roman emperors? What was the signing of the peace treaty to end the third crusade? Were the Tudors in the Middle ages? First person who wore the first wrist watch? Where does the name steimel come from? What was one of the most important but indirect and unintended impacts of the mongol drive to the west? How many days since December 8 1959? What was something new that contact with the East introduced during the crusades that caused life in Europe to change? Where did the first civilization occur? How did the crusades help spark the Italian Renaissance? 100 years is how many centuries? What calendar year is the same as 1995? What rights were townspeople given in medieval societies? What was the primary motive for European exploration in the 15th and 16th centuries?