The outer wall of the citadel town is perhaps its single most important feature and is one of the most impressive found anywhere. It is this perimeter wall which surrounds the town that gives it its fortified look and dominates the modern City of Erbil. The wall is a continuous ring of about a hundred houses of various vintages.
Perimeter houses, that are houses built on the outer edge of the top of the mound, are structurally the most vulnerable. Some of the houses have collapsed over the last 50 years either due to subsidence or underground water seeping from leaking piped and sewerage. Obviously, the collapse of an outer house creates a gap and endangers adjacent houses. A house collapsed as recently as 2006.
Houses built on the outer perimeter of the mound had to face the structural challenge of stabilizing the foundations of their outer wall. Therefore, retaining walls that tapered from some 1.7 to 1.2 meters were employed. Some larger houses of notable families had semi-circular buttresses or protruding brick features that had recessed arches and also used as terraces.