I saw interesting thing
Go to Tavern -
you'll see that you can cash in your gold
How much gold is already cashed in / How much gold you can MAX cash in
Now write the number that will be bigger than MAX,
and a message returns:
Fortress Vault capacity has been reached. Fortress Vault (upgrade at Defensive facilities) influences the max gold that you can cash in into the Tavern.
So, can we hide there our gold?
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And Fortress vault description:
Automatically stashes and hides the resource amount its level capacity allows, while the province is under attack. Each level increases the capacity by 10%, given the basis 2000.