Splintert has made an interesting mod for DynaCulture that allows you to control whether settlements should assimilate gradually.

When a settlement is founded, it has to choose its own kingdom and there are some factors that can help or hinder the process of assimilation. The mod has a number of settings that can be used to control these factors. The first is the kingdom's influence over the other kingdoms. By default, all of the kingdoms are influenced but you can set it to only the one you want.

The second factor is the influence of the owner of the kingdom. The more influence the owner has over a settlement, the more it will shift to a new culture. By default, the owner is given a sizable increase to influence but you can set it to be a smaller number.

The third factor is the influence of the neighbors. By default, the distance is set to include geographically close settlements within a few hours travel time. But you can set it to be a longer time.

The fourth factor is the assimilation resistance factor. It is a number that determines how fast a settlement will shift to a new culture. It is set by default to a number that will see a settlement stabilize over a few days to a week of in-game time. A higher number will slow down the assimilation.

The fifth factor is the linked settlements cause influence. It is a number that determines how much influence a linked settlement has over its neighbors.