I can recall there was a topic like this. But instead of a territory it was a regional castle.
I like the idea. But i think those accomplishment should be "guild" accomplishment.
I would go for the claiming territory by capturing the regional castle( perhaps that is what you mean? ) but it should be a guild accomplishment only so you really need 4 guys and you can reinforce the regional castle with any troops from people in the guild. Once in the regional castle it can't be pulled back only the guild leader or officer can give the order to send the troops back.
What you think about this?