Have you read the docs? The information you need should be here: http://assault.cubers.net/docs/mapediting2.html
If I remember correctly, you should be able to make the arch from your mapping menu when pressing Esc in-game, and then use heightfields to modify them.
Mod edit: link fixed
If I remember correctly, you should be able to make the arch from your mapping menu when pressing Esc in-game, and then use heightfields to modify them.
Mod edit: link fixed