Castle Story ~ how it all began

Science doesn't merge to well with a early medieval setting so I recommend you set it in the late middle ages.
Alternatively you could make it be after the collapse of a big country in the modern day which would probably lead to a period that was more "medievaly" (because the collapse of Rome led to the middle ages) in order to retain a little of humanity's technology.

 

I don't know, this is still in VERY early development right now. I might just cover the Ballos part in the beginning cut-scene and make the actual game take place on the island when you and Curly go to destroy the first wearer of the Demon crown.