I see a lot of historical and fictional illustrations of walls that do not have wall walks. My understanding of medieval tactics is largely based on cinema and some wikipedia, so as far as I know a wall is mostly only good for standing on top of, to fend off attackers using ladders. In my mind, when I imagine a wall without a wall walk, I see people scaling the wall with ladders, with the defenders in a courtyard now in a lower vantage point as people pour over from above. This seems less than ideal, so I assume I'm missing something.
Does a wall without a wall walk only serve as a very preliminary defense, or a privacy screen, rather than a defense against a true raid or invasion (or anyone with a ladder)? Or is there some other tactical advantage that I'm missing?