The proxy jump was done by supercharging into a wall and getting hit by an arrow.

Here's the video:

Skip to 2:43 for the start of the proxy jump glitch.

I decided to have a go at it, and while I couldn't get high enough to reach the same building that was reached in the video, I still managed to get on top of the other castle easily.

Here it is:

I do still want to get on top of the castle in the video, and when I do, I'll edit this topic, and bump it if it takes me long enough to get on top of the castle that I haven't got to yet.

The challenge is to get on top of both castles.

Also, please could a moderator or an administrator change the name of this topic to Lofty Castle Challenge., with the full stop?