The Norman adventure in the south is a great story and Normans served as mercenaries in the Byzantine army as well as against them. Most famously in 1081 when they invaded the Empire in modern day Albania at dyraccium (modern Durres).
Not just a great story but also a good wargame army. Heavy knights with a ferocious charge supported by a solid spearman base and quality missile firing with bows and crossbows. I like the Sicilian version that gives you light horse archers as well.
These are from the Gripping Beast range.