Vault and everything about it
The Vault protects a portion of your resources in the event that you are attacked and loose. The storage capacity of the Vault increases every level, without any relation to your other resource buildings storage capacities, allowing you to protect more resources.
At level 1 your vault protects 10k of silver, but when you upgrade to level 2 it can no longer hold silver until it reaches level 21, and for level 2 to 20 the Vault only protects your food, lumber, iron and stone resources. When your Vault reaches level 21, your silver is also stored in and protected. Gold is always safe and protected.