A really foolproof way to hide stuff.

There is no reason to let an attack plunder your city if you are on line. Just remember that all an attack does, no matter how large, is take out your fixed defenses, and take any resources that you leave lying around. The fixed defenses is a nuisance, but there is no reason to give them resources.

As I have explained before you should ship as much stuff as you have transports and troops out of your city with a camp time that puts it past the attack. Just ship to a close valley, the camp keeps it from being exposed. Then bring it back when it reappears in your close valley.

If there is more than you can ship, then sell all of it in the market. This will immediately give you gold. Turn right around and buy something like lumber with all your gold. Jut make sure the transaction goes through. It takes 30 minutes to be delivered. During that time it is completely safe. Just do this before the attack, and all your stuff will show up after the attack. You can then sell the lumber and buy whatever you need.

If you are going off line, then ship your resources to a nearby friend, or sell for gold and do the same thing.