The Mongol Empire is the largest land empire the world has ever seen. At its height, it stretched from Siberia down to the Middle East and from China to Eastern Europe. The Mongols committed some of the worst atrocities in human history to conquer their empire, but once their empire was conquered, it facilitated trade in a way unseen previously in human history. The fact that all territory between China and the Mediterranean was effectively governed by one administration ensured that trade could flow freely and people could move safely as never before.

Genghis Khan conquered the largest empire the world has ever seen. His empire stretched from China all the way to Central Europe and from Northern Russia down to the Middle East. It was the first incidence in human history where the massive stretch of land between Asia and Europe could be considered somewhat unified and organized. This connected Europe and Asia into one cohesive trade network for the first time in human history and helped facilitate the exchange of ideas, produce, and disease across the entire landmass.

Meritocracy is the name given to a system in which promotions are made and jobs awarded based on the ability of an individual rather than the status of his or her family. Genghis Khan famously introduced this concept to his Mongolian forces, greatly improving the efficiency of his forces and the loyalty of his men.

Trebuchets are siege weapons that became a popular component of medieval war-making in the twelfth century. A trebuchet is similar in design to a catapult, although significantly more complicated and effective. Trebuchets were eventually phased out in the sixteenth and seventeenth centuries with the advent of gunpowder and cannons.

The invention of gunpowder happened in China sometime around the ninth or tenth century. The China of this time period was a highly innovative society that gave the world many notable inventions, including block printing and compass navigation at sea. Gunpowder spread across Central Asia and the Middle East before arriving in Europe around the thirteenth century.

Charles Martel was the ruler of the Frankish people in the early eighth century C.E. He is most famous for defending the realms of Christendom from Islamic invasion in 732 C.E. when he won the Battle of Tours. Islamic armies never reached further north than Spain in Western Europe. Without Martel's victory, it is very possible that all of Europe would eventually have been Islamic instead of Christianity, and the whole history of the world would be very different.

Lay investiture is the name given to the common medieval practice whereby the Holy Roman Emperor, or another secular ruler, would appoint church officials without the consent of the Papacy. Gregory VII saw this as a challenge to his rule and banned the practice. He also subsequently excommunicated Emperor Henry IV when he ignored this rule. 

The schism between Eastern and Western Christianity had been developing for several decades before it was finally formalized in 1054 (the eleventh century) with the excommunication of the Patriarch of Constantinople by Pope Leo IX. From this point on, Christianity was divided between a Catholic West and an Orthodox East. The main cause of the schism was the centralizing and autocratic power of the Papacy in the eleventh century, which was challenged by the imperial power of Constantinople. This was the first of many subsequent divisions of Christianity that have occurred in the centuries since.

Simony is the practice of selling (or buying) positions in the Catholic Church. Although technically forbidden under church canon law, it was nonetheless extremely common throughout the early medieval period. It was an important part of various controversies throughout the Medieval period and contributed to the loss of faith, among certain parts of the European population, in the Catholic Church.

The Spanish Inquisition was founded in 1478 by the significant Spanish monarchs Ferdinand and Isabella. It was originally intended to judge the legitimacy of those Jews and Muslims who rather than choosing to flee Spain (both groups were banished from the country) instead opted to convert to Christianity. The Inquisition was set up to determine if these conversions were false or not. Later, the Inquisition morphed into a device to wield state power and ensure uniformity of religion through torture and execution. Contrary to popular belief, it was rarely used within Spain to torture Protestants simply because there were so few Protestants in Spain. The Spanish Inquisition would last, in some form or another, until the early nineteenth century.