Causality is a part of this because causality is a product of time. Without time, causality is meaningless. So, since time exists not in things in themselves but rather in our own understanding, causality is not a function of things in themselves but rather a function of our own reasoning.

Does that make any more sense? It is hard to imagine time as something separate from things, because our minds are literally incapable of conceiving of the world in that way. That's the whole point- time and space are the preconditions our mind needs to understand the world.

This distinction is actually a bit of a shortcoming in Kant, because if you can say there is a world that we can't access by way of perception, then you can start to make all sorts of suppositions about it, which is exactly how we get into these weird New Agey theories. If you're going to criticize the theory, go for that part of it, because it is definitely a weak part of the argument.