The Gesta Guillelmi is full of echoes of classical texts, but it is difficult to be certain what library sources William of Poitiers had at hand when he was actually writing it. The Gesta frequently makes a close comparison of the British campaigns of Julius Caesar with the campaigns of William the Conqueror. Most of the echoes of other classical sources could have been remembered from his student years; the occasional phrases and aphorisms are of the kind that memory most readily retains. If he finally settled in Lisieux and was writing there he would have had the resources of the cathedral library to draw upon.
Although he may have commenced in writing of William’s Norman campaigns at any time after the conquest, most of the evidence points to a date after 1071 for the bulk of the writing. He certainly wrote after William of Jumieges had completed his Gesta Normannprum Ducum. It is believed that in the planning of the Gesta Guillelmi, William of Poitiers was strongly influenced by classical models, and to a lesser extent by the shorter accounts of dukes assembled by Dudo of Saint-Quentin and William of Jumieges.
On the whole, the originality of the Gesta Guillelmi suggests that it is above all a book of memoirs, written by a man of letters who had been well drilled in youth in such of the classics as were then available, but had spent his mature years nearer to the seats of power, both secular and ecclesiastical.
Even in its incomplete form, the Gesta Gulillelmi is the extended biography of any duke of Normandy. It was planned after 1066 to demonstrate how Duke William prepared for, and achieved, the Conquest of England; and to justify his succession to the throne.
In an early chapter describing Earl Godwine’s responsibility for the murder of the aetheling Alfred, William of Poitier refers to the retribution that was to come with the defeat and death of Godwine’s son Harold.
He divides his work into two parts – that of the deeds of William the duke and those of William the king. William of Poitiers is writing from memory some twenty years after the event, but the themes are important.
In his later years, Orderic wrote that William of Poitiers was forced by unfavourable circumstances (unknown) to abandon his work on the Gesta Guillelmi,which he would have continued until the death of King William. He gave himself up to silence and prayer, and composed sermons and verses. Evidently he lived until after 1087, the date of the king’s death; but whether failing health or a fall from favour forced him into retirement is not known. However, it is worth noting that William of Poitiers connection
with Bishop Odo is consistent with his fulsome praise for the bishop, and may help to explain why a panegyric dedicated to the Conqueror was never completed. Consequently, Odo’s connection with Robert Curthose, whose first rebellion against his father began in 1077, and his later disgrace and imprisonment, must have caused many of those closely associated with him to fall from favour.
His accomplished Latin style and his thorough familiarity with a wide range of classical authors are clear proof that he studied for several years at Poitiers before returning to Normandy. Something of William of Poitiers character and ability can be deduced from his writing, as he used Sallust as well as Caesar for battles, Marcus Cicero and St Augustine for moral dissertations, and some of his knowledge of legends of the Trojan War may have come from the Ilias Latina. He was certainly familiar with Norman customary law as it was enforced by the dukes, and was aware of some at least of the different English customs. As a Latinist, William of Poitier is superior to the chroniclers of his day. Orderic Vitalis marvelled his Latin prose and abilities. Sources used by William of Poitier consisted mainly of histories of the dukes of Normandy by Dudo of Saint-Quentin and William of Jumieges.