The floor is one row of lower half-slabs, the back of that wall is entirely flat, the underside of it is dug out, now if you look closely you'll see that the birch logs are placed diagonally form the fire block.
Fire spreads diagonally though, so why is it not doing it here? Is this really a safe setup or is it only a matter of time before something burns? (I tested this for over 30 minutes without anything burning)
Why does this work? Is it really safe or just a fluke? I've seen similar fireplace designs fail.