Final answer:
William the Conqueror is likely the one who started building a castle when he joined the server, constructing a motte-and-bailey as part of his military preparations.
Step-by-step explanation:
The person who started building a castle when he first joined the server could be referring to William the Conqueror. Immediately after dining, William and his half-brothers Odo and Robert plotted their military strategy. One key part of their preparations was the construction of a motte-and-bailey fortification. The motte-and-bailey consists of a keep (tower) on a motte, which is a raised earthwork, accompanied by an enclosed courtyard, known as the bailey. These structures showcased the military provisions and tactics of the eleventh century. Additionally, the use of drystone technology reveals the skills and methods used in constructing enduring castle structures during historical periods.