You can't even access this location until you receive the Wyrm Hunt quests from Ser Maximilian....the quest that allows you to reach this location is called, "A Fortress Besieged" at the Shadow Fort. Once you get through the hole in the ground, up the stairs, and out the door, you will be in a large "courtyard" with two cyclops and a bunch of goblins. There is another hole in the ground on the right side semi-close to the actual fort. (If you open your map, you should see some "brown circles", the hole you want is the top left one.) Go down the hole, advance a little bit, and the cheevo will pop!!
There will be a pawn guild bulletin board quest later that takes you there as well, and unlocks the door at the end of the tunnel.