In the countryside, well-maintained dry stone walls are a marvellous habitat for wildlife. You can try to emulate this in your garden, on a smaller scale.
Dry stone walls are built without using mortar, and the nooks and crannies this creates makes them valuable for wildlife. As the wall ages and matures, gaps become a bit larger and bigger animals can also use the wall.
In fact, when a new wall is built, the first thing that is likely to happen is for it to become colonised by lichens. Soil accumulates, making it possible for plants such as mosses, ferns and pennywort to colonise. This adds more mini-habitats that different animals will find suitable.
The best walls for wildlife are neither too new and nor too old (when they are likely to have started collapsing). They can provide shelter to a wide variety of creatures. Even a small patch of moss on the wall can support a variety of invertebrates. The wall itself can hide several different species of woodlice, as well as springtails, beetles, and snails. Spiders lie in wait in the cracks, and various species of bee, ant and butterflies may all use the wall.
A full-blown countryside drystone wall needs to be well over 60cm (2 foot) high to be valuable to wildlife, and probably extends some distance. It is unlikely you will be imitating this directly in your own home, but it is useful to follow some of the basic principles.
For maximum wildlife value ensure that at least some of the wall is in shady, even damp, conditions.
Whatever the size of your wall, have a wider base and taper it as it goes up. This will give added stability.
Start the base off by digging a trench for the first layer of stones.
Start with larger stones at the bottom, and where necessary infill along the base with smaller ones.
Build the wall up on two sides. Lay the stones in layers, and in each layer make sure each stone’s long side is facing into the wall (rather than along the length of the wall). Stones should touch below and to the side and should be as horizontal as possible.
As you build up the sides, make sure to fill in the middle with smaller rocks.
Finish off with a neat layer of coping stones at the top, making sure that they are in close contact with the wall.
You can follow these instructions for a basic section of wall, or adapt them for a much smaller version.