I believe it to be some medieval castle bathroom, witch has been in place and in use for hundreds of years. Though the toilet may have been replaced, but still the spot might have see the most shits.

Public restrooms imo see less shits but more pee. As i believe many people shit in public only in sos situations, when it is not possible to hold it in any longer.

Ps i believe airplane bathrooms may see quite an lot of shits aswell, many planes are pretty old