For residents of a castle, when nature called, the garderobe was the place to go...the curved structure is the exit hole of the garderobe and the small arrow loop above doubled as source of fresh air for the users.
Even centuries after its last use the stain and erosion caused by countless numbers of humans reliving themselves multiple times a day for more than five hundred years is clearly evident.
The advantage of relieving oneself behind the protective walls of a castle were obvious. The problem that arose was where the waste ended up, piled up on the side of the exterior wall (as in the case of the above garderobe where the exit was angled to direct the waste further away from the wall), or in the moat. In the former case the human waste was most often collected by peasants to use as fertilize in the local crop lands. Some lords even charged their peasants for the use of the waste. In the latter case, having your waste entering the water table was a danger the residents of a castle were probably unaware. Besides, at that time period drinking ale/beer was safer, gave need calories and was probably more enjoyable.