Title is basically a TLDR for the following:

I am creating a fantasy world for a DnD campaign, and I want the organization of the main populations to be like the feudalism but in a smaller scale than what happened in our world. The land already known to men during feudalism was - forgive me if I am mistaken - huge, almost all of europe, right? But I want aspects of this world to be unknown to men while also having kingdoms big enough that they can wage war against each other, but small enough that they don't have enough influence to discover the rest of the land. Another question: What would be the obstacles that a kingdom would face so that it can't expand quickly to the other parts of the continent?

If you guys have any tips on what I should aim for in my fantasy world, that would be great! Also, here is a sketch of the world map: http://imgur.com/a/oX2Rm

If you made it this far, then thank you very much! And sorry if I made spelling mistakes and etc, english is not my first language haha