a)This period was called the Dark Ages mostly due to the presence of many wars. This led to many deaths. Also, conquering and fighting other places led to the spread of diseases. It also had the Black Death.
b) The fall of the empire was mostly due to it being so large, and not having 1 ruler. It split into 2, and was soon getting attacked by the Gauls because the Huns were pushing them into Roman territory. Then the Huns themselves attacked Western Rome. Eastern Rome survived after being attacked by the Ottoman Empire. It turned into Byzantium.
d) The feudal system was the system in which the Middle Age people were categorized. The top was the king, and he was the richest. The bottom were the peasants, who were the poorest and consisted of the most people. In the middle were knights, who then served the upper class of lords and nobles.