Stair construction indeed plays its part
OK my thoughts behind it
Ever cycled in the countryside ?
Ever came to a flight of stairs put in for the tourists by those wonderful people at the forestry commission ?.
Were the thick sections of timber they use put on top for you to stand on or were they on their side with a support on each end and the void filled with earth.
Now look at the supports on the side of these stairs :?
Where is the force pushing, against them? or away from them?
So these stairs are simple planks supported at each end and filled in behind. I say planks but could be hewn stone
Further musings put the pic in a rural country. A very basic trial and error solution to building stairs,probably Mk2

The mosaics make me think of eastern Europe.