To keep field voles and moles away, we recommend placing a wire mesh on the bottom of your structure. It is not strictly necessary, but it will help protect the roots of your plants.
Cover the bottom of your structure with a layer of clay pebbles. This will help create a draining system, which is necessary regardless of the location of your garden. If you skip this step, the roots of your plants may be damaged by excess water.
Another layer should be made of mulch (bark, twigs, wood chips), which will improve yield and provide additional protection of soil and roots.
Compost, i.e. organic green waste, such as shredded bushes and leaves, will make a perfect subsequent layer for additional nourishment of your plants.
The top layer should be composed of topsoil. This choice is up to gardeners themselves, as everything depends on what they are planning to grow.