Long Long ago, giant snails roamed the world. Being slow they were generally not considered a threat and therefore tolerated. Sometimes a snail of great Age (and size) would pass away. moving slowly to the great cabbage patch in the sky and some enterprising individual would find the remaining Shell and move in.
A bit of work was necessary to carefully remove the central spiral "walls" but after this was done a cosy waterproof abode could be constructed.
1/87 bod from Prieser, the cat from an Outcast-Figures set (available at the Croeburn 1813 shop)