I'e been writing an AAR about the d'Hautevilles on another forum. Here's the post I made detailing the background history: while it's based on various sources including that Wikipedia article, I think it gives a good concise and coherent summary:
Robert Guiscard and his brother Roger.
Background and History
The Norman conquest of England is well-known. Rather less well known is the Norman conquest of southern Italy. Bands of marauding French Vikings set themselves up as feudal lords in the hot, dry lands of the Mezzogiorno, defying Emperors and Popes alike. This is their story.
About the year 1015, a Norman named Osmund Drengot murdered his kinsman. As punishment he was banished from the Duchy of Normandy; he and his four brothers made their way south, eventually winding up in Italy. They gathered a group of about 250 motley adventurers as their followers, acquired a base in the mountains, and...
Well, the official histories say they protected pilgrims and kept the roads safe for travellers, in return for a modest fee collected from those they assisted. Of course that really means they were bandits.
They were successful bandits, however. Over the next ten years their army grew larger, both with local recruits and from reinforcements coming from Normandy, where other people had heard about the Drengot brothers’ success.
Then in 1027 a war between the lords of Capua and Naples left the former Duke of Naples, Sergios, without a realm to govern. He turned to the Normans, now under the leadership of Osmund’s brother Rainulf, for help. Norman warriors put Sergios back on his throne and in gratitude he made Rainulf the Count of Averso. The Normans were now respectable, and a power in the land.
Italy just before the Norman arrival
A new group of Normans arrived around 1035, in response to an appeal from Rainulf for more of his countrymen to come to strengthen him. Once more they were led by a group of brothers: the many sons of Tancred d’Hauteville from the Cotentin region of Normandy. This is my dynasty in the game. The d'Hautevilles fought as mercenaries in the many wars of the region.
In those days the Byzantine Empire still claimed rule over most of southern Italy, but its power was challenged by rebellious local lords. A large revolt in 1040-42 was joined by the d’Hautevilles and their followers, and when it ended William ‘Iron Arm’ d’Hauteville was ruler of the lands around Amalfi. There were now two Norman dynasties in the region – the Drengots and the d’Hautevilles would mostly be allies of each other.
When William died in 1046 his brother Drogo took over, and extended Norman power further through Apulia and Calabria, at the expense of the Byzantines. However he angered the Pope, and was assassinated in 1051.
A third d’Hauteville brother, Humphrey, took over next. He was unable to calm the hostility of the Pope, who in 1053 sent an army to punish the d’Hautevilles. They allied with the other Norman dynasty, the Drengots of Aversa, and defeated the Papal army at the battle of Civitate. Pope Leo IX was tossed into a Norman prison.
Humphrey died in 1057, and brother number four took over: Robert Guiscard (me!), who was about 42 years old and had greatly distinguished himself at the Battle of Civitate. Robert continued to spread d’Hauteville power in Apulia and Calabria, while his allies the Drengots conqured Capua.
In 1059 the new Pope Nicholas II reversed the policy of his predecessors and allied with the Normans in return for support against the Holy Roman Empire. Richard Drengot, the Count of Aversa, was confirmed as Prince of Capua. Robert Guiscard received even greater honours: Duke of Apulia and Calabria – with Papal blessing to become Duke of Sicily too if he could conquer the island from its Muslim rulers.
First Robert had to complete the conquest of Calabria and Apulia, which still had Byzantine garrisons in places like Reggio, Taranto and Brindisi. This was finished by 1060, but a Byzantine counterattack the following year delayed the invasion of Sicily until 1061. In 1061 Robert and his youngest brother Roger captured Messina, giving them a foothold in Sicily.
This is where the game begins (in a 1066 start).