We often picture ancient cities as being surrounded by thick, solid walls for protection. There are certainly examples of this from ancient times, but the heavy expense plus the significant construction time led to an alternative city wall design that we call a casemate.
A casemate consisted of two thinner, parallel walls with empty space between them. It was faster and cheaper to build, and afforded the inhabitants extra living space in crowded cities. During times of war, the space could be quickly filled with dirt and stones to create a thicker, solid defensive wall.
The Bible account of the conquest of Jericho suggests Rahab the prostitute’s house included a casemate.
She let [the Israelite spies] down by a rope through the window, for her house was built into the city wall, so that she lived in the wall. (Joshua 2:15, ESV)
The Elah Fortress/Khirbet Qeiyafa is surrounded by a casemate wall. I had the distinct pleasure of helping to remove the stone/dirt fill from one of them in the last days of this season’s dig. The rocks can be quite heavy.
Solid stone walls are stronger, but the heavy use of resources or other constraints restricted their use at times. Solid walls were sometimes reserved only for vital cities such as capitals.