King of Seas

You will need to progress the story far enough to take settlements and then complete a mission called "Defensive strategy" that gives you access to the 'governor/cove' option when you're at port. You'll have to do this at Eagle's Den because you can't level other settlements to max until Eagle's Den is maxed. It costs approximately 450,000 gold to max out Military, Population and Wealth. Achievement will pop once you buy up the max ranks.