Hey Guys, thought I'd pop on and clarify this. Basically Ituralde and Magnum pretty much have the jist of it (although Magnum, since turns are sequential not simultaneous there's no danger of two factions being able to both build the top level).

Your actions attract guilds to your settlements, though all your settlements earn guilds separately, on any turn a faction can be offered a maximum of one guild / chapter (to avoid spamming). If you choose to take it and can pay the cost the guild will set up in the settlement.