Modding/adding special loyalty/culture buildings

[caravel]

I do this as well, and it really is the "second best thing". But I'd like to see invaders have a much harder time than myself in controlling my provinces. This is only fixable trough region+faction specific loyalty buildings, I think. But maybe you meant that Cynwulf has already implemented this (I haven't, as of yet, I might add)?

This can't be done. The rate that a province "acclimatises" to it's new owners is hardcoded as far as I can tell. The modifier to this is religious difference. The only real way to do this is to increase the rebelliousness of a province. This means that a faction has a harder job in holding it down.

It is worth noting that the AI is poor at this anyway, and provinces such as Portugal tend to host a lot of faction reappearances. The way to counter this is to have a low base loyalty but at least give factions the means to pacify a province. This can be done by adding happiness bonuses to certain buildings. Currently only the Church, Town watch, Watch Towers and Brothel have this effect, but it could be extended to other buildings and upgrades, such as those to the Town Watch and for the Tavern and it's upgrades for example.

Happiness upgrades could also be applied to the castles themselves. In this way building a larger castle would serve to better stabilise a province. Makes sense really.