Aside from Harrenhall, which castle do you think has the best defences (man made or natural) and why?

Tie between the Rock and the Eyrie. In both cases, you need to either surround – in which case your lines are going to be stretched across miles, and it’s still extremely difficult to starve them out due to the size of their storehouses – or assault a mountain, in which case you’re either going to be engaged in tunnel-fighting or trying to fight your way up the side of one of the tallest mountains in the world.