To prevent moisture rising from the ground and up into our walls, it was necessary to lay a damp proof course (DPC) after a few layers of brick and stone. These days this is typically accomplished by using a plastic membrane, but as our materials brief stated:
We want to avoid synthetic materials wherever possible. The ideal house would biodegrade over a couple of hundred years of non-use.
We therefore went with one of the traditional methods of damp proofing which is to lay a double layer of slate (the second layer offset from the first and so covering the cracks between slate pieces). The slate we used is second-hand Welsh slate. As you can see in the final image below, after the DPC was laid, further courses of brick and stone were put on top to complete the plinth walls.