Why? For a 4 kyu challenge one would think you should be able to add a class on your own.

Sure, but it's just more convenient. People are here to solve some algorithmic/math tasks, not to learn how to create a class in python.

Could someone explain me, please, when do i need to return "Entry denied: citizen of banned nation." I just couldn't get the logic for this case.

What was the reason of changing the level of this kata from 2kyu to 5kyu?
I think that this kata is much more harder than average 5kyu kata on this site.