This square structure has toilet 'seats' along the back walls with a roof above. Although some wealthy citizens had private home bathrooms, they also used the public toilets; some even paid a membership fee to claim a specific seat. Turning into the structure's entrance, you’ll note a small aperture; here stood the clerk, who collected fees from visitors. While the whole experience was indeed a public one, the flowing Roman toga would have provided a modicum of privacy.
The rest of the room was open to the sky, with a floor covered in mosaics (visible though the wooden boards you walk on). 'Toilet paper' in ancient times was a natural sponge on a stick soaked in a vinegar solution.