A guild owning a castle may not participate in other castle sieges outside of theirs, on offense or defense.
As said in game when trying, "The guild possessing a castle cannot register for castle attack & defense"
This thread is meant to discuss this mechanic, personally I have mixed feelings about it.
Some unknown factors: Guild levels and alliance system(?) Can an allied guild defend an ally, even if they have a castle? I am in favor of this, personally. Remember the game says up to 5 defenders and up to 25 attackers, although it seems attackers are pitted against each other, maybe not if they are allies?
Being able to only join one of the sieges per week is rather disappointing. It also is a huge strain on gathering Battle Points(Located at the bottom right of your screen), which is what the political system rankings seem to be based on.
The reason this is in place is obviously to prevent a monopoly of all the castles, which I agree with. I do question not being able to participate though, at least as a defender. Being reduced to one siege per week as a castle owner seems like a reason to not own a castle, as you will fall behind in Battle Points and less action in a pvp based game.
Hopefully a good discussion comes out of this with some good opinions!