just a guess

No footpaths or streets were used between the dwellings, which were clustered in a honeycomb-like maze. Most were accessed by holes in the ceiling and doors on the side of the houses, with doors reached by ladders and stairs. The rooftops were effectively streets.

• this would protect houses from any animal that can't climb a ladder or the wall itself (also if they pull the ladders up, from anyone who doesn't have a portable ladder)
• if houses are constructed in clusters, that would mean one less wall to build (maybe 2)