This photo shows the retaining wall complete with posts for the roof supports and rubble fill.
The reason for pouring the slab is to keep the oven from settling unevenly. Since the rubble may still move as it settles, the slab makes sure the oven doesn't crack as this happens.
Wine bottles are set in clay slip perlite mix to insulate oven so that heat doesn't escape into the rubble below.
Starting the center line of the bricks on the leveled sand before completing the fire brick floor.