In the early period of Ottoman history, yes they did. In the fourteenth and fifteenth centuries many Christians fought both directly in the Ottoman army, and also as auxiliary forces under the command of the Christian princes who were Ottoman vassals. We find record of Christians serving as timariots (prebendal cavalrymen) in the Ottoman army as late as the time of Mehmed II (r. 1451-1481), and they may have lasted in small numbers until the early sixteenth century. Christians continued to fight in the Ottoman army under vassal princes (Wallachia, Moldavia, and Transylvania) throughout the sixteenth and seventeenth centuries.