Eruv (atys)
A reason may be that the current alliances between the nations are not hammered in stone. According to the Lore there have been e.g. Tryker-Fyros alliances in the past, and there have been some Fyros-Zorai quarrels just recently. why not 2 civ aligned per guild? zora/fyros tryker/matis ?
A national guild with members from other nations would have troubles when an alliance breaks, or worse in case of a state of war between the nations in question.
I would consider it appropriate when neutral guilds were allowed to recruit citizens, because national guilds can take neutrals. Anyway, there are reasons not to allow that which seem justifiable as well.
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Daomei die Streunerin - religionsneutral, zivilisationsneutral, gildenneutral