"An honorable establishment this is not, and yet, some of these lowlifes aren’t evil. In fact, all but a few pray on monsters in dungeons, rather than well guarded cities." In game description.
Thieves' Guild is in the same location in Outer Terra as Fierce Fold/Dripstone is found in Taskmaker, but the map layout is totally different. You can reach it from the South by cutting through the mountains, or from the North by taking a ship from Zehner's Palace across the lake. In both cases you will appear in the same place on the map.