The old tower, also known as the "Tower of Debauchery" is the most prominent architectural feature in the camp followers' encampment. Beneath it are a number of chambers.
The underground chambers can be accessed in a number of ways. One way is through a trap door in the back of the prostitutes' tent, by presenting Madame Carole with a square coin and asking for Whistling Wendy, then giving her the pass phrase "I want your smile to part the gates of paradise".
It also happens to be the hiding place of one Vinson Traut.