discuss the major development of the mediaeval period
Answers
The Medieval period lasted from about the fall of Rome till around 1300 AD. We are talking 900 years here, give or take. I assume you are speaking of Northern and Western Europe.
The Dark Ages, or early Medieval period was a time of a loss of knowledge, of migrations of Eastern and Northern European barbarians in Europe. Meanwhile, things were not quite as bleak in the Mediterranean and Middle East.
I suppose any trends of the Medieval period must originate with the Muslim/Christian conflicts. Islam kept alive much of the knowledge of the ancient Greeks and Romans, and added to it the knowledge of more Eastern Cultures. People tend to forget this because of the militant nature of Islam, but along with conquests they brought knowledge. The Crusades exposed many Europeans for the first time to a bigger world. I personally think that the Crusades had a lot to do with the rise of the merchant class in Europe of the 12th and 13th Centuries. The rise of this merchant class, and the spread of knowledge from Eastern cultures to a somewhat backward West led in turn to the Renaissance.