According to Daniel Goffman in The Ottoman Empire and the Early Modern Europe, how did the Ottomans administer their vast realm, and what did...

In the late 14th century, the Ottoman Empire began to use slaves as part of the janissary corps, or infantry units that made up the sultan's bodyguards and the troops that served in his house. These soldiers served the sultan directly under leaders such as Bayezid I, Mehmed I, Murad II, and Mehmed II. Janissaries were taken away from their cultures and became loyal to the sultans. They helped, as Goffman writes, "domestically to neutralize...

In the late 14th century, the Ottoman Empire began to use slaves as part of the janissary corps, or infantry units that made up the sultan's bodyguards and the troops that served in his house. These soldiers served the sultan directly under leaders such as Bayezid I, Mehmed I, Murad II, and Mehmed II. Janissaries were taken away from their cultures and became loyal to the sultans. They helped, as Goffman writes, "domestically to neutralize the Turkoman cavalry and internationally to neutralize European innovations in military technology" (page 65). Therefore, the janissaries were important in helping the sultan consolidate power at home and abroad.

The janissaries formed one part of the kapikulu, or slave military corps that composed a great deal of the Ottoman bureaucracy. Following Mehmed II, nearly every grand vizier was from this class. By the time Suleyman was in power, one had to be part of this class to even be considered for advancement. Membership in what Goffman calls this "pseudo-aristocracy" was determined more by being a slave than by ethnicity, language, or any other quality. Not only did the aristocracy own slaves, but many of the aristocracy also were in fact slaves. 

While there were different classes of people in the Ottoman empire, Goffman writes that "there was considerable variation in the memberships and meanings of these groupings" (page 91). He writes that this adaptability was essential to the Ottomans' ability to rule over such as vast area:

"The secret to Ottoman longevity and the empire's ability to rule over a vast and mixed collection of territories was not its legendary military, its loyal bureaucracy, its series of competent rulers, or a particular system of land tenure. Rather, it was simply its flexibility in dealing with this diverse society" (page 92).

The Ottomans dealt with different classes of people differently in different areas of its empire, and exercised what Goffman calls "flexibility in governance" (page 92). These were the keys to its ability to rule over a vast area.