Oh you don't even have to ask ! Of course you can change the coastline... even the places of mountains, lake, rivers, etc... The main thing for me is to feel that the kingdom is fortified. Whatever you need to arrange in the geography, don't even ask and just do it, you have full licence on the geography of this land
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On that part, I must be blind, cause I couldn't see the places where paper and ink were damaged... I was probably too happy to see what you achieved so far to see the damaged parts ??? I really love your colour tests and the display of the whole map : border with celtics designs, color of the sea and land, the way you play with the border and landmasses ! It's all good for me ! (really good, actually !)
If you prefer to switch to digital, it's OK too : we all know your skills with computers... but if you want to go on with paper, in my opinion, you have more than a good start with what you are showing here !