Balbus built a wall

and so did I.

As you can see, intense concentration is required:



but the end result is pretty good: everything pretty well level (or vertical, depending), and solidly made:



It's a 20cm thick wall. When it reaches the beams above (another two courses), it will provide some support for a new beam, and take a lot of the weight of the new hearth. As the hearth is to be roughly 3.5m by 1m and will be concrete 15 or 20cm thick, the wall needs to be quite robust. The plan is to lay the top row of blocks upside down - ie with their open faces on top - and fill them with concrete and a few reinforcing bars to tie them together.