Well I assumed it could be done for castles since the author of the mod "Expanded Japan", (link at the end of post), created new cities and factions, those cities had to have their own battle map, otherwise they would not be playable. If there really isn't a way to make a castle/city battle map and use it in the campaign, then there goes half the reason to make a map in the first place. for me, who prefers campaigns over zero-reward custom battles, making a map holds no use unless it can be applied to campaign lol. but i suppose thats the way it goes
i hope someone can figure it out and show me how
i built a kick ass capital for tokugawa clan and named it Kaminotoshi, short for City of Gods! haha