The landscape looks artificial (and in the river valley nonrelevant) and just by that very original and interesting.
The somehow grey zone on the canyon that looks like a coiled shell is actually a huge city. A maze of buildings that evokes the feeling of really an old toen grown for a long time. It works by itself along the game. The element which provides the structure of the settlement are the whote walls, a massive combination inside as transition from the keep to the lower city, and a line of city walls, built pleasingly and with eyecandies but rather incovenient.
So, this mission is about defending. But not with all the measures which such a well-equipped economy may provided. You have no quarries or pitch rigs at hand. And from the units, crossbows and spearmen are disabled. What's left then to keep an enemy at bay who will invade in big numbers?
I remember a scene in a movie about Joan of Arc (called "The Messenger") where the English shout, "They must not reach the walls." Very ptobably, you will find your melee troops in the forefield of the outer ward, too, engaged with the enemies. That's the rare experience.