Islam
FW does not really focus on Islam, so there are relatively few rules governing how to play a Moslem.
Islam is a few centuries old now, and is no longer voraciously expanding. Now, Islamic society is fractured, and Moslems spend as much time (or more) fighting each other as they do conquering new lands for Islam. In fact, in 1007, three different people claim the title of caliph! Still, Spain and Sicily remain Islamic, and Moslems do still occasionally try raids of forced conversion into Europe. The Roman Empire (we call it the Byzantine today, but its contemporaries call it Roman) stands as a bulwark protecting Christendom from Islam. The Crusades have not yet begun — but will soon, so tensions between Christianity and Islam are high.
The Moslems control the most advanced society in the world (possibly tied with the Chinese). In terms of education, science, technology, and law, Christian Europe can't compete. Christians, however, have a very Eurocentric view of the world. Most of them have no idea what Islam or Islamic society is like. They simply assume that Europe is the civilized center of the world.
The Moslem world is a very important trading partner for Europe. Europe has a very low level of technology, so almost every item that is a luxury or technological innovation comes from or through the Moslem world.