Does my alliance stay the same from one reign to the next?

Yes. The alliance is the game's persistent group — it survives reigns and World transitions.

That's what sets it apart from the rest of the social structure:

• The faction is reshuffled every season (you're placed among 3 camps, balanced, never guaranteed).
• The Great House is purely cosmetic.
• The alliance stays: these are the players you carry on with, reign after reign.

Concretely, when a new reign starts — often after a server merge — you find your alliance and its members again, even though the map, your faction and your territory have changed. So that's where to invest socially: pick or build a solid alliance, because it's the one thing that follows you and coordinates your conquests season after season. The alliance is the real group; the faction is just the stake of the moment.