could someone explain dukes and princes to me?

[Kosez]

In some Slavic languages a ruler is called a ˝knez˝ or ˝knjaz˝, ˝knaz˝, etc... And principality is called ˝knezevina˝ and similar. Aristocratic titles go like this: King, Prince (a heir to the king),
Knez (ruler of principality),
Duke (in some languages Vojvoda, Vajvuda, etc..),
Count,...

Principalities had a different meaning in Slavic Middle Ages. They tended to be quite large entities, sometimes also independent.

So I guess difference between principality and duchy could sometimes be that principality was not in feudal relation with a kingdom or empire?

 

[Mongoloid Cow]

Every feudal ruler in the Middle Ages had a title. But they didn't just get to choose their title, whether they were independent or not. The modern principalities of Monaco and Liechtenstein, or the Grand Duchy of Luxembourg are testament to this. All titles had to be based on a claim and a birthright. eg: the Duke of Savoy, despite being independent for all intents and purposes, could not just up and declare himself a king - he instead had to obtain a kingdom first (Sardinia, then Sicily).

@Kosez: no, the difference is the rank and the title. But the ranks of Dukes and Princes are reversed in Slavic Europe if what you said is correct.

 

[sydhe]

That's what remained of the Byzantine Empire at the time, pretty much only Constantinople (which the Ottomans took in 1453).

They also still owned a good chunk of the Pelopennese, also shown on map.