I was impressed by the integrity of the inner layer (pictured below) and the outer layer (long gone in this photo). Curiously the middle layer (pictured above) -- which should have been the structurally most sound - was very crumbly.
Look how uneven my oven floor had become. The sand beneath the bricks sifted through the gravel bed and caused the floor to cave. This had been a constant headache for me in the early months after the oven build. I had to remove the bricks and relay gravel and sand, but it was impossible to make it level after the oven structure was in place. For the latter years of the oven, I lived with an uneven floor. Remarkably, it did not seem to matter as much as you might think. Nevertheless, I have vowed not to let this happen again. This time I will lay a bed of concrete for the sand to sit on.
The rebuild has begun. I added a course of stones to the base and added crushed adobe from the demolition as extra fill.