The public toilets were made so that people could speak to other people at the same time as they were there. They were not separated by walls and you did not need to flush the toilet as it was a continuous amount of water to clear it away. These were made of stone so they must be cold.
Romans were not the ones to invent the arch but, thanks to their improvements they developed them so that they could be used on huge structures. They were then capable of building domes with that. They used arches in the Coliseum and so it helped to support the heavier surfaces. This was at the bottom of the circle where the people did whatever they were supposed to.
Roman architects were great builders. The buildings were not important from the outside but inside they were made to be very spacious and comfortable.
I left the Anim Gif as a joke. I will find images of Roman homes.