Siward is a real historical figure who opposed William in the years after the Conquest. He is rumored to have been captured by William, then released and banished from England.  Here’s a little bit about him:  The story goes that in 1075, a Siward, earl of Gloucester led a flotilla from England into the Mediterranean. Onboard was the flower of England’s native aristocracy, disenfranchised by the Norman Conquest of 1066. They reached Constantinople, gaining permission from the emperor to settle in Byzantine lands on the Black Sea’s northern shore — if they could reconquer them.

They did, and founded a Nova Anglia, the towns of which echoed the names of the ones the settlers had left behind. No trace of the colony remains today, but this New England (on the Crimean peninsula, and immediately to its east) must have survived for at least a few hundred years. Well into the 14th century, the New Englanders’ annual Christmas greetings to the emperor were conveyed “in their native language, English.”