A tip I have always used when creating maps, I always start with a blank canvas

ie A flat map

I then play a few battle on this terrain, find out where both sets of troops start from and work from that basis.

ie I build terrain to suit the armies starting positions, hense the defenders always start in my castle.

I created a great map for Calais castle only to find that the defenders never started the battle in the castle, after a bit of deleting and adding new stuff they always start in the castle now.