Information is a curse and a blessing. On the one hand, having real prototype information makes our models that much richer. On the other hand, once you have it, it cannot be ignored.
In my case, Peter Newman shared a Del Rosamond photograph of the roundhouse after it had burned (unfortunately I don’t have permission to share it on), and it clearly shows that the joists ran length-wise in the stalls. This is somewhat unusual, and means that the arches joining posts ran across the stalls, rather than conveniently lined up on straight walls.
This configuration means in turn that cutting the lintels for those arches was a labour of cut and fit. The posts are all rather stuck in their holes, and pulling the roundhouse off (at least the first time) is going to be a challenge. So, I added the knee braces, and will add the first joists before attempting to lift the model from its base. I’ve left the front wall loose to enable me to work on the doors, which I guess is next.