A spiral herb garden is a creative way to make use of very limited space. And, as an accent piece it is a nice focal point of the garden. It's simple to construct. It's simply bricks placed in a spiral. There is no mortar involved and the soil is put in and around, then planted in a spiral like fashion. If you have poor soil this allows you to build on top of the underlying soil. By going up you have better drainage for the plants. The idea is to put the plants at the top that need the best drainage. Thus the rosemary and lavenders are at the top, then as you go around the spiral and down there are salvias, different thymes, oreganos, basils and dill, then at the bottom the mints that need to be contained because they are garden bullies but can handle wet feet so they do quite well at the bottom. Because it is built up and a soil mix used it doesn't matter the soil underneath. If you can't go down, go wide or tall.