In the 4th century A.D., Rome was on its way to total disillusionment. The Eastern Empire became the Byzantine Empire, with Constantinople for its capital. The West fell to barbarian invasion, and the emperors lost their power. The fall of the leaders in the West led to the rise of the church, and in the West we see widespread construction of churches in this area during the Early Medieval Period. (I'll get to the churches in a bit…)
The Medieval timeline is split into three parts: (1) Early Medieval, (2) Romanesque, and (3) Gothic. The Early Medieval Period starts with the fall of Rome. Strong, central government is gone; the ruling influence during this time is uncertainty, conflicts, open warfare, and apparent chaos. Feudalism is pretty much the only system of order, and it entails a system in which weak noblemen gave up their lands and much of their freedom to more powerful lords in return from protection. Serfs were the poor peasants who had no land.