The Golden Key is one of the first major sub-quests the heroes of Dragon Quest II face after acquiring a ship. The search eventually leads to the village of Zahan. The previous owner of the key apparently died on a fishing voyage, leaving only his dog knowing the location of the key. By speaking to the dog, he leads the heroes to the specific tile which should be searched to find the key.
The Golden Key is one of the most beneficial key items in the 2nd half of the game. It provides access to numerous excellent pieces of armor (Tectonic Plate, Erdrick's Helmet, Erdrick's Shield) as well as the village of Burrowell where more keys can be found.