The safety concern about the highest rung is, in this context, hilariously inappropriate to the high risk of death involved in the activity.

It is considered not safe to step or sit on the top rung of a ladder as you can fall off. Most ladders have markings saying not to step on the top 2 rungs. But I imagine ladder safety is not much of a consideration as you're trying to scale a castle wall during a siege.

I think the idea is that we can see the impending accident, when the guy on the ladder reaches the step, which is not a step, and falls down?

Personally, I find the humor in these comics to often be quite difficult to understand. But even so, I always like it when they pop up in my feed!