Inside the townhall is a seagull. Next time him is a key, try to pick it up. The seagull will offer a trade for 25 gold stars.
After you get the key go to the sewers, under the flag in the middle, at the bottom is a gnome sanctuary. The key goes in the door which leads to the portal.
You will get this achievement shortly after entering the portal, you may have to walk a few feet in.