How do you get a gold shield?

This shield is a Doomsday skill. You can find it in the resource specialty (green), top tree.

It's called "Resource Cellar" and its description says: "After the base is successfully sieged, 100% of the base resources will be protected from being plundered, lasts 20min, 24h CD". (This is the text from level 1).

Skill will cost you a minimum of 18 specialty points (to build the tree all the way to level 1) to a maximum of 22 points if you wanted to max it out.