The player alliance model will mimic the de feudal model.
Each player may choose to be a vasal of another player, basically joining an alliance lead by that player.
However, every player may have their own vasals, as many as they want, becoming their sovereign lord.
This creates a hierarchy. In the hierarchy, each player may choose to renounce the vasality given to their sovereign and choose a new sovereign, or none at all.
This move happens with that player retaining all its own vasals, so, basically it will move the entire group.
Players will receive one ore more castles when they choose to become vasals, as well as permission to access the lord's lands for PvE content. Renouncing a lord means renouncing also all these benefits.