Two thoughts:
Does the ground there ever freeze in the winter? Probably not, but where I live, foundations need to extend below the frost deptg.
What weight can the undisturbed soil bear? Even if you aren't fighting frost heaves, stone walls are heavy, and over time, problems can arise. It might be worth pouring a concrete footing to better distribute the weight.
In both cases, perhaps consulting a structural engineer familiar with your local environment would be in order.
Excellent suggestions; the foundation is the most important.
Those are good points. We dip below freezing but not much, so heaves won't be a problem. What could be a problem is the weight, like you said. I am concerned for shifting. The best thing would be to compact the ground, which I do not plan to do, because well I do not have one, and maybe I could rent one.... I might consider it.
I plan to pour concrete in sections for the floor and press flat smooth stone into it. Which risks of cracking. Lots of research to do. First, I have to remove the pesky tree in the middle of my spot!