One of the only good books on this subject is Mark C. Bartusis 'The Late Byzantine Army: Arms and Society, 1204-1453'. Even though the 15th century is more recent to us compared to the rest of Byzantine history, evidence of what their armies were actually like during this period is fairly fragmentary.

However, from what we do know, late Byzantine armies were fairly unimpressive affairs. Unfortunately, in contrast with the well-trained, equipped and organised armies of the thematic system of the middle Byzantine period, the armies of the late Byzantine period were quite ad hoc.

Late Byzantine rulers adopted the self-defeating attitude that Byzantines made better farmers than soldiers and so were reticent to pull them off the land. Obviously they still raised levies of militia when required for defence, however these were usually poorly trained, equipped and organised, relying on formidable Byzantine fortifications to hold out against the enemy until they withdrew. Late Byzantine rulers never invested the time, effort or money into developing and maintaining a quality army.

On the rare occassion that late Byzantine armies went on the offensive, foreign mercenaries (most often western Europeans, but sometimes Turks or others) were prefered. These generally performed better than native armies, although their loyalty was always dubious and they were quite expensive. Considering how poor and decentralised 15th century Byzantium was, only small contingents of foreign mercenaries could be afforded (although sometimes larger bands were hired and not paid, causing them to either desert or start pillaging Byzantine lands).