Casemate walls:

Building a stronger wall more cheaply by constructing two parallel walls a small distance apart, and joined at intervals provided many of the advantages of a large solid wall, however it was much less costly in materials and labour. Such a wall is known as "casemate" (probably from the use of the word to describe a kind of picture frame).

Where the space between the skins was large rooms could be incorporated, as at Beersheva (see diagram, based on Herzog). Such walls were believed by Yadin to be typical of 10th century Israelite construction, however their choice seems more complex and to have depended on economic and convenience factors.