Cities on Dwilight can suffer badly under the new immigration code. Apart from births, they rely on the surrounding townsland to provide immigrants. If that townsland is under populated I would assume immigration can be rather slow. For Rettlwville to grow at its maximum potential you will need to hold Rettlewood and some other surrounding regions in order to secure a steady immigrant supply. If you can grab some regions that border fully populated regions, then they should grow even faster and that should trickle through your realm as well.
That would make a future colony of Darfix almost impossible unless Astrum first expands towards the city.
