What were Eastern and Southern Europe like in the medieval period? Did Southern and Eastern Europe have knights and castles like Western Europe? I've never heard for example of Italian knights in the crusades. All I know that eastern Europe was under Byzantine influence for a long time. Specifically I'm looking for information on Romania in the 15th and 16th centuries?
Confining my answer to the area around modern-day Romania (IOW: The north-east Balkans).
In the beginning of the middle ages, this area was sort of a borderland between the territory of the Huns and of Eastern Roman Empire. It kept this state for another hundred years after the mid 6th century, except that the Huns were replaced by the Avars, and the Eastern Roman Empire became what we know today as the Byzantine Empire. At the end of this period, Slavic tribes began to move into the depopulated Balkans in large numbers.
By the mid 7th century, a (likely Turkic) tribe, the Bulgars, were defeated by the Khazars and driven into this area. At first Avar vassals, these Danube Bulgars eventually threw off the yoke, and set up their own empire in the area, known as the First Bulgarian Empire. At its height in the late 9th century, it encompassed nearly all of the Balkans, save the coastal Byzantine areas. This is also the period when the Slavs got their alphabet (and thus literacy in their own languages became a possibility again) and when the people of Balkans were converted to (mostly Eastern Orthodox) Christianity.
After this the Bulgars went into a long period of decline, during which their territory got slowly squeezed out between the Byzantines and the Patzinak Turks. By the mid 11th century, the borders of the two met on the Danube. Not much later another Turkic tribe, the Cumans, took over the Patzinak's territory. They held it until the Mongols arrived in the late 13th century, and took it over.
Meanwhile south of the Danube, in the early 13th century the Venetians convinced the leaders of the 4th Crusade to expend their efforts within the Byzantine Empire itself, essentially dismantling the Empire. This allowed the Bulgarians to reassert themselves in the area south of the Danube. This second Bulgarian Empire lasted until the end of the 14th century.
At that point, all of the Balkans south of the Danube was conquered by the Turks. The area immeditately north of that was being run as the Principality of Moldavia (with chiefly Vlach nobility) from the mid 14th century until it was also conquered by the Turks at the very end of the 15th.
That pretty much brings us into the modern era.
Italy was one of the key participants, money and and troops provider for the crusades. Italian mercenaries, the Condottieri were employed throughout Europe. Italy was the leading producer of body armor in Europe (followed by Germany). Politically Italy was fragmented into rivaling city-states and the papal area, time to time subjected to the Holy Roman Empire.