Most sloping garden sites can be made more attractive by creating muliple levels and the most common way is by low retaining walls.
By low, I mean about knee high (600mm) or maximum thigh high (900mm) - anything higher requires a bit more effort and skill.
And of course any wall over 900mm high requires council approval - it will also be classified as "Structural" rather than "Decorative".
You can use hardwood, like Oregon or an easier, lighter and usually cheaper alternative is treated pine. This has been "Tanalised" - heat and pressure treated with anti-rot chemicals and will last longer than most people live in a house for.
Cantilever Facing Walls:
These are easy to make from round logs - usually 100-150mm diameter held with galvanised spikes or nails.
Start by digging post holes around 100-200mm deep and fix your uprights - usually leaning a few degres back into the area you will be backfilling.
You can fix them in place with either rammed earth or re-fill the hole with a concrete dry mix, add water, mix with a trowel and leave to dry overnight.
Next lay the round logs behind the upright posts and wedge into place with some spare pieces of wood.
Use a spirit level to ensure they are level - these kind of walls can be built parallel with sloping ground but generally look better if they are level, even if you have to have an uneven number of logs of different lengths to achieve it.
When you are happy with the look then you can fix them securely to the upright posts with galvanised spikes, nails or screws. Don't use ordinary nails or screws, as they will rust in no time.
Saw Toothed Walls:
This type of wall is quick and easy to build, as it has no uprights embedded into the ground - its strength is in the right angles you create. Slabs or rounds are best for this type of wall.
Prepare the ground level before you start and just lay the logs on top of each other, securing with a galvanised spike or nail as you go. It is however important to ensure that the dimensions of "A" are at least 1/3rd the dimension of "B". This internal overhang provides the anchor strength for the wall once the wall has been back filled.
Regardless of the type of wall you build its best to backfill the wall with rubble, gravel or blue metal right up next to the timber with the soil behind it. This allows the new raised level to drain properly without the fine soil particles clogging up the natural gaps between the timbers.
N.B. Most treated pine logs are guaranteed for anything from 25 - 40 years but it does depend upon NOT cutting the timber pieces.
When they are treated, they are cut to size first and the chemicals are forced into the timber under heated pressure but do not necessarily penetrate right through the entire piece.
If you cut it you will probably reveal part of the timber core that has not been treated and can therefore provide an entrance for bugs or fungii that will weaken the timber.