The Tower of Penitence serves as a prison for those awaiting execution under the powerful Soukyoku (its the name of the execution grounds). The Tower is heavily guarded, and within it are contained very slim windows for the sole purposes of the prisoners to gaze at Soukyoku before their execution.
Its exact location is not known, most likely somewhere within the Sereitei. Inside of the academy it looks like a modern school crossed with a Japanese castle. The rooms are high with tiered seating as well as large halls that seem to go on forever. Within this school are many shinigami-in-training as well as very well respected teachers. This is the same school from which Renji and Rukia graduated.
During the Soul Society Saga there were a number of people that were arrested and thrown in jail. There is a section of Sereitei that is solely dedicated to the detainment of certain individuals. The cells themselves are very generic with their vertical bars as well as the cot where the prisoner would sleep in. Usually, they would be constrained with spirit handcuffs that cut off the prisoners reiatsu supply.
The sewers are like a complex maze that twist and turn. It is likely that if you don’t know your way you’ll easily get lost. The 4th Division is the only division that can easily navigate the labyrinth as they use it as a supply route to get from one are to the next easily. On top of that the sewers are quite clean seeing as how the 4th Division only deals with the “chores” of The Sereitei.