It specifically says the Lower level of Undead Burg. To access this area, you need to find the Basement Key which is on the steps in front of the church by the gate near where the Fang Boar was. Once you have the key, you go to the bridge where the Hellkite Wyvern is and the door leading to Lower Undead Burg will be at the end near where you meet Solaire. Once you head through and down the tall ladder, you can go up the stairs to your right to open a very convenient shortcut from the main Undead Burg bonfire to Lower Undead Burg. That is the staircase the page is talking about.