Y18 contains snow and scattered rubble. Additionally, they seem to have come up against "building" 173: A large, thick metal wall running along the easternmost edge of the downtown area. It's far too slippery for anyone to even hope of climbing. Y19 contains the edge of a building, but it can't be entered from here. Y20 contains buildings 157 (the building they saw before) and building 161 (a much smaller building). What will they do?

As for specific information about the wall, it's some sort of impossibly slippery metal. It could be steel, but it could be something else. It's tall enough to stand level with the snow walls, and sounds like it's at least a few feet thick.