"The Mint produces gold. Upgrade it to boost its production and bank capacity."
The Mint produces gold and stores it until collected by the player and placed into a Bank. When the mine is full, production will be stopped until it is collected (or raided by an enemy player).
The production rate and storage capacity depends on the level of the Mint. One can estimate the amount of resources in the mine by looking at the cart at the right side of the mine.
The Mint takes up a 3x3 space.
When you choose to upgrade a Mint, it will automatically collect any uncollected Gold. Note that if your Banks are full at that time, the uncollected gold will be lost! During the upgrade the mine stops producing gold.
Once the Mint is level 5, you can activate auto-collect function for 3 days.
Attackers can steal up to 50% of the uncollected Gold in your mine. Having too many uncollected resources will make you a more attractive target.
If a Mint is destroyed in an attack while it is being upgraded, no gold will be lost, as the mine is empty during this time.
Pay careful attention to the location of Mints when planning your attack, as Mints are usually only marginally protected and can be quite lucrative, especially at high levels. Many times a single Musketeer can be placed in such a way that it can destroy a Mint while standing clear of any defenses.
Although round numbers can occur in Banks, they happen much more often when Banks have been emptied by a previous raid and the resultant shield has allowed the Mints to completely fill up.
A downside with Mints is that even though the cost to upgrade them is low, the time it takes to complete them is relatively long.