After you are back out in the open [South Quarter / Riverside - Eel's End] return to the main path and go east, while watching out for the single guard with a torch, who may be busy patrolling this area. After you reach the fork, do go neither to the North, nor to the South. Instead, climb onto the rooftop of the building of the building to the east, shown in the above screenshot, and walk along the narrow path between the buildings. The upside of this method is that you will be able to approach your destination without any problems, without worrying that you may be spotted by someone.
This path will take you to the brothel entrance region, and you can tell it by the red lamp on the right. Also here, you should take your time, because there is a opponent taking a look here, on a regular basis. Wait for him to leave the area and jump down, from the rooftop, afterwards. You should notice that the access to your destination is blocked off by the lifted plank. The solution to this problem is, fortunately, very easy. You need to explore the area on the right and interact with the lever shown in the first screenshot above. You can now walk over the plank and open the door to the undergrounds, which will start the third chapter of the game.
Note- before you start the third chapter of the game, it is worth making sure that Garrett is prepared for that, when it comes to his equipment. I recommend that you take several Rope Arrows with you, and make sure that you have the Wrench and the Wirecutter on you. Having the abovementioned items on you will allow you to access the paths, which are inaccessible in any other way, and reach unique loot.