How would you stop a Man-in-the-Middle attack in a Fantasy setting?

Inspired by The Wandering Inn's latest chapter, what would you do If you are the 5th Wall's Bastion-General? Assume you have all the resources that the Kingdom Rhir provides, the irregulars from the other kingdoms and the Champions that was isekai'd from Earth.

Backstory [here](https://wanderinginn.com/2020/07/12/7-34-c/) and [here](https://wanderinginn.com/2020/07/15/7-35-c/), actual attack [here](https://wanderinginn.com/2020/07/19/7-36-c/). Don't worry, these all happens at the literal other side of the world from the main characters, and can be read as a standalone three part story.