What causes a city to become a colony?

[midget_roxx]

I'm playing on a continents map and I've recently taken another continent nearby from another civ. However I've noticed I get these popups saying that these colonies are costing me maintenance and I should give them autonomy. I've got other islands nearby that don't have the colony status so I'm wondering what is the trigger for a city being a colony

And would it be worth releasing the colonies just for a bit of extra money?

[happyturtle]

There have to be at least two cities on the same island before you can emancipate it into its own colony. When you do this, a random leader will be chosen for them, and they act as a vassal. They have a +10 diplomatic modifier for "You gave us our independence" so they start out friendly. Of course you'll get more negative modifiers with the other AIs for gaining another vassal.

One thing to watch out for is that they'll start out knowing all the techs you do, and there's nothing to stop them from trading them around to everybody. So if you have techs you want to guard, don't do it. If you want control of the cities, don't do it. If you need them to be the same religion as you for diplomatic purposes, make sure you've actually spread your religion there, or they might catch a random one from an AI.

When you release them, they'll gain two garrison units per city, but they won't have any other units. I usually gift them a couple of workers and a ship or two to give them a boost up. They'll usually start to fall behind you pretty quickly, with only a few cities, so you can't count on them researching anything for you. They're happy to let you have their resources though.