If explaining your sewer system is necessary, then by all means do so. But if you stop the progress of the story or scene from moving forward, then don't do it. Just hint at the system and let the readers use their own imagination...

As to piping, most sewer piping systems in medieval times was baked ceramics. Metal was too expensive and the technology too inferior in that the metal pipes would rust out in a very short time causing more expense to dig up and replace.

You also have to remember, most buildings did not have indoor plumbing or potties, even in big cities. Castles and large estates had limited indoor plumbing and potties. Whatever system is in use, they have to be large enough for someone to go in and unclog, as well as clean it out from time to time.

If you research sewer systems in ancient European and Mediterranean cities, you will see a lot of them had very large drainage tunnels that led out to a river or sea. It was just one of the causes of the plague in how the rats could travel freely.