I think your mental model of a castle is not quite correct. It's not necessarily a simple stone box. A lot of roofs lead to higher points, or to walls.
Your mental model of what can happen in a castle is also not complete. It's not just sieges, there are any number of reasons someone could be chasing you. It could just be one assailant.
Finally, perhaps I didn't describe the step well enough. The step is not the top one, it is the second one down, and the face of it is around 2cm higher than all the other steps. It wouldn't make any difference to the drainage. Don't get angry that there are features of castles that are not adequately explained, and some people try to explain them.
Your mental model of what can happen in a castle is also not complete. It's not just sieges, there are any number of reasons someone could be chasing you. It could just be one assailant.
Finally, perhaps I didn't describe the step well enough. The step is not the top one, it is the second one down, and the face of it is around 2cm higher than all the other steps. It wouldn't make any difference to the drainage. Don't get angry that there are features of castles that are not adequately explained, and some people try to explain them.