I think it's A nucleus because it is the control center of it
Since Roman was expanding and it didn't have control of their lands and couldn't govern most of it. It kept expanding until the western part of Roman was conquered by Barbarians.
It was important for them to be able to stop the muslim armies from advancing farther into western europe.
History Learning Site
The Black Death of 1348 to 1350
Citation: C N Trueman "The Black Death Of 1348 To 1350"
historylearningsite.co.uk. The History Learning Site, 5 Mar 2015. 19 Apr 2018.
In Medieval England, the Black Death was to kill 1.5 million people out of an estimated total of 4 million people between 1348 and 1350. No medical knowledge existed in Medieval England to cope with the disease. After 1350, it was to strike England another six times by the end of the century. Understandably, peasants were terrified at the news that the Black Death might be approaching their village or town.
The Black Death is the name given to a deadly plague (often called bubonic plague, but is more likely to be pneumonic plague) which was rampant during the Fourteenth Century. It was believed to have arrived from Asia in late 1348 and caused more than one epidemic in that century – though its impact on English society from 1348 to 1350 was terrible. No amount of medical knowledge could help England when the plague struck. It was also to have a major impact on England’s social structure which lead to the Peasants Revolt of 1381.
