Instructions: In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Input: Passage: Victorious at the Battle of Hastings on 14 October 1066, the invading Duke of Normandy, William the Conqueror, spent the rest of the year securing his holdings by fortifying key positions. He founded several castles along the way, but took a circuitous route toward London; only when he reached Canterbury did he turn towards England's largest city. As the fortified bridge into London was held by Saxon troops, he decided instead to ravage Southwark before continuing his journey around southern England. A series of Norman victories along the route cut the city's supply lines and in December 1066, isolated and intimidated, its leaders yielded London without a fight. Between 1066 and 1087, William established 36 castles, although references in the Domesday Book indicate that many more were founded by his subordinates. The new ruling elite undertook what has been described as "the most extensive and concentrated programme of castle-building in the whole history of feudal Europe". They were multi-purpose buildings, serving as fortifications (used as a base of operations in enemy territory), centres of administration, and residences.William sent an advance party to prepare the city for his entrance, to celebrate his victory and found a castle; in the words of William's biographer, William of Poitiers, "certain fortifications were completed in the city against the restlessness of the huge and brutal populace. For he [William] realised that it was of the first importance to overawe the Londoners". At the time, London was the largest town in England; the foundation of Westminster Abbey and the old Palace of Westminster under Edward the Confessor had marked it as a centre of governance, and with a prosperous port it was important for the Normans to establish control over the settlement. The other two castles in London Baynard's Castle and Montfichet's Castle were established at the same time. The fortification that would later become known as the Tower of London was built onto the south-east corner of the Roman town walls, using them as prefabricated defences, with the River Thames providing additional protection from the south. This earliest phase of the castle would have been enclosed by a ditch and defended by a timber palisade, and probably had accommodation suitable for William.
Output:
What is the title of the person who sent an advance party to prepare which city for his entrance?