You can go to this ruin from
Zaphgott Moor => Plitzerback Wetland => Arctus Ruins
You get the gold key from this place. The key is located on B3.
It’s located on Arctus Ruins B3. Near the Mutant Hellion’s room (Naught).
==Metallic Funerary Mirror==
It’s located on B3, you need to obtain the gold key to be able to open the door.