View of the interior wall of one of the excavated houses. The walls, and the ceiling, are made out of black basalt (volcanic) stones. Note on the right of the image the "interior windows" which must have helped a bit with light and air circulation. On the top of the wall note the basalt slabs, that in turn support the paving slabs. A very massive construction indeed.
For a map and a brief description of Katzrin Click Here.