Agree with isaiah, specially because a two way arrow is commonly used with other purposes. So I bet people won't read the instructions seeing the arrow but instead they will come here to ask why they can't attack when there's a two way arrow on the map.
From the day in which bombardaments were introduced, they were only mentioned somewhere on the map....honestly I think it's enough.
So, without considering the bombardment issue, that is more a "lazy" players issue than anything else imo, there's something else that is not clear or not balanced on this map?
Give every man your ear, but few thy voice. Take each man's censure, but reserve thy judgment.