Stan rogers wrote:Owning a port will give up to 30% discounts on shipbuilding and level ups and 10% additional trade revenue.
A port population contributes to a nations tax base that in turn raises a nations treasury that can be accessed by a countries council
You must be a citizen of a country to be a recipient of its treasury.
This makes multinational guilds difficult to disperse fleet or trade assistance to guild members that require or deserve it as Guilds have no ability to maintain a treasury.