This is a secret location that can only be visited after going through a long series of events.
However, if you started with Witch, Cock of Demonic Origin, or Inhuman Ancestry, or if you are a follower of the Old Gods, then the location is permanently unlocked. You can Take a Walk to increase Constitution and Joy. You can also go to the ruins directly once you have done step 3 below.
Although quests aren't officially denoted in the game, getting into the ruins involves one of the longest series of events in the game, besides those for endings, so we'll call it that. This quest involves rescuing Sareth, a slave that you meet at the Main Slave Pens, from the Evil Mine. Every Slave can undertake this quest.
- Get your slave to 50+ obedience, then visit the Docks. A second location at the docks, Main Slave Pens, should now be available.
- The first visit to the Main Slave Pens is a dialogue choice. Pick "Promise to help" (#2). If the first option is chosen, Sareth's location won't be revealed in the next step.
- Continue visiting the Main Slave Pens, preferably if your slave has a Teddy Bear. Eventually the story will continue and unlock the third docks location. (Tifa can also locate the slave described while doing Some Milking, and the Ruins will not be revealed.)
- After you've seen all of the necessary events at the Main Slave Pens, the Secure Slave Pens will become available. Go there until you have an event where Lady Farun is being raped by a monster. Rescue her yourself and then Visit her. During your first visit, ask for permission to visit the ruins.
- Continue visiting Lady Farun until she asks for your help. Attend Court and then Visit her again. She will ask you to get some Nymph's Tears for her.
- Now I know there are different ways to do this, but this is the easiest I have found: Visit Astrid (in the Forest), and she will tell you about her supplier of Nymph's Tears. Walk around the Lake or Palace after that to find the Private Tutor, who will make an offer for the Nymph's Tears; say yes. If you've taken Nymph's Tears from the Private Tutor already, you can skip Astrid and go to her directly.
- Visit Lady Farun again and you'll get your pass to the ruins. It will now appear on your map when you Take a Walk.
- No help is at Farm, Forest, Astrid, Town Center or Slave Market
- Approaching the Palace will get no assistance, and, from then on, the Count and Lord will refuse to speak with her during a Visit (even if she has 100 Reputation and they have declared their love for her)
- A group of Thugs can be hired from the Slums for 50 GP but they seem to have no effect during the Fight
- The Bounty Hunter can be hired for 300 GP from the Port Area if you have met her before, or for free if your slave has befriended her
- The Lake water will ward from evil, increasing Defense
- At the Seer, the mercenary Misana can be hired for 1000 GP to automatically win the Fight for you
Losing this Battle will lead to a Bad Ending (if enabled) or redoing the Fight again tomorrow. Winning will complete the quest and hide the mine location again (if you don't have it permanently unlocked). You will also recieve a Bit Gag, Harness, and Vibrator Panties if you do not already own any of these items.