The defeat that stands out the most to me occurred fairly early in my Byzantine campaign, against the Mongols. They attacked Trebizond with a force of about 600, including a lot of heavy cavalry, with some horse archers and foot warriors mixed in. I had about 900 men, mostly Trebizond archers and Byzantine infantry, with a couple units of spearmen and Kataphractoi. The two generals were roughly evenly matched, although I think mine had a slightly higher rank.

I arranged my infantry in a defensive semi-circle on a hill, with the Trebizond archers inside the formation. The heavy cavalry approached my position and spread out in all directions. They attacked straight up the hill on my center, left, and right all at once - they didn't try to get around my refused flanks, they just attacked head-on. They cut through all my defending infantry with ease; neither the spearmen nor the Byzantine infantry slowed them down much. The infantry line collapsed, and the archers were soon fleeing in terror. My general fought bravely, and eventually left the field without running away, but most of his army was killed or captured. Losses on the Mongol side were relatively light.

The same Mongol army marched against Constantinople that same year, and I barely managed to save the city - mainly through the stupidity of the Mongols.