The Coin Vault increases protected coin quantity. It requires Base level 15 for construction, later than the Food and Iron Warehouses.
How coin protection works
The vault provides 300 protected coins at level 1. This rises to 60,000 at level 10, 3.3 million at level 20 and 75 million at level 30.
Each number is the protected quantity at that building level. It is not coin income, and upgrading the vault does not deposit that many coins into your resources. The increase from an upgrade is the difference between the two levels.
Coins come from a different building
The Gold Mine produces coins per hour. The vault handles protection, while the mine handles supply. Their stats have different meanings even though both concern coins.
The Food Warehouse and Iron Warehouse protect their own resources. Technologies can also improve protected resource quantities, providing a separate source of bonuses from these buildings. Construction costs still need the actual coins, food and iron required to start the project.
