Boom Beach Economy guide: Vault
While what is essentially a glorified safe may not sound thrilling, the Vault is one of the best additions you can make to your Home Base, and the buildings I'll prioritise the upgrade of over all others when given the choice.
When you slap one of these on your island, a percentage of your total resources will be protected from raids by other players.
This is invaluable since it means you can react to raids more quickly by building defences quickly to patch holes, you can build units to retaliate, and you don't have to start from zero every time you lose in a battle.
Gold and Wood can be stored initially, but after a few upgrades the Vault will also hold Stone and Iron, which are much more valuable.
On top of all this, the Vault acts as additional storage too.
Seriously, the Vault is awesome.