Defensive gameplay and other tactics.

I don't understand how to get my units onto the walls.

Archdemons (promodernized, alt-upgrade) have a special skill that allows them to teleport units. They can take another unit and move it to the desired cell on the map. I'm attacking a castle, my warriors approach the walls, and then this nuisance grabs one of my units and moves it to its location, and my unit starts getting attacked from all sides on the wall.