You can capture a region by your alliance controlling at least 80% of all habitats inside a region at the time of Region Update. Hence, every member has to help! Castles, fortresses and cities each count as one habitat.

Once your alliance controls at least 80% of all habitats in a region during the Region Update, the region status will change to “pending”. It will be captured only if you control enough habitats at the next Region Update and you will be able to upgrade it.

Region Reset 

Once a region changed its owner, all its upgrades are reset. Therefore, whenever you capture a region, even it was yours beforehand, you have to start over upgrading it. 

So make sure you don’t lose control of a region, especially if it is completely developed. On the other hand, you can weaken your enemies heavily when they lose a fully developed region!