This wall is just part of the outer wall surrounding this dwarven city. The mountains create a natural wall around the city, and so this portion of the wall is separate from the main gate. I added fog at the edge of the map, allowing the enemy forces to get closer without getting blasted by cannons, fireballs, and archers. In my case it'll be magic fog summoned by the enemy, but use it how you'd like!

Throw some siege towers in and boom, you got yourself a big battle.