Lower Undead Burg is a transitional area that moves you from the main (upper) area of Undead Burg to The Depths. It’s a really straight(forward) area, but the enemies still pose quite a challenge. The Undead Attack Dogs and Hollow Thieves are fast, nimble enemies that deal extra poison damage. The area’s boss, the Capra Demon, is a brutal fight in a confined space (and there are two Undead Attack Dogs in there as well).
There are several useful items along this corridor and a couple of new armor sets. If you purchased the Residence Key from the Undead Merchant in (upper) Undead Burg, you can unlock the room where Griggs of Vinheim is held, where you can also find the Sorcerer Armor Set. Further along the corridor, you can find the black leather Thief Armor set.
Once he’s freed, Griggs will head to Firelink Shrine where he’ll sell you sorceries and sorcery accessories. In the aqueduct, you’ll meet the female Undead Merchant. She’s got a limited selection, but the mosses she sells are invaluable when dealing with poison and bleeding.
There are no bonfires in this area, but there are entrances and exits that are all close to other bonfires. Once you use the Basement Key and climb down the ladder from the bridge (close to Solaire), you can run up the nearby stairs to unlock the shortcut to the (upper) Undead Burg bonfire. At the other end of the Lower Undead Burg, you can find and follow the aqueduct that will lead you back to Firelink Shrine.