A terraced or tiered landscape provides an ideal solution for heavily sloped yards. Adding a tier will aid in preventing soil erosion, slow down water runoff and will add a bit of visual interest to a garden. Terracing can also allow for slightly different soil in each section, so a wider selection of plants is available, giving the terraced landscape a feel of several small gardens in one space.
The particular material for a terrace will vary greatly depending on the local weather conditions and overall look you desire. Pressure treated wood is a common choice, as it's highly durable, diverse in color and fairly inexpensive. Modern pressure treated wood requires very little maintenance, though older varieties require fairly frequent resealing. Products manufactured prior to the 1970s were coated with Chromated Copper Arsenate, a potentially hazardous chemical that contains chromium, copper and arsenic. Instead of wood, you can try brick, stone or concrete. Most stone or masonry materials are more expensive than wood.
Your terrace's wall height will depend largely on the steepness of the slope. Terraces should be tall enough for the ground in each tier to be level. Anchoring the walls is an essential part of proper construction. The anchors must be deep enough to withstand the freeze/thaw cycle, and to hold up against heavy rain and wind. The weight of wet soil can be heavy enough to cause bowing or the collapse of improperly constructed walls. Most local building codes provide outlines for these projects, and must be followed to prevent costly repairs and fines. Walls more than 2 feet high should be built by a professional because the walls will have special drainage considerations and anchoring needs.
Planting and growing on a slope is a difficult, often awkward prospect. Tiering, when done properly, alleviates some of the awkwardness by evening out the ground. Keeping tiers properly proportioned will not only be aesthetically pleasing, but will make them easier places to work. Keep proportions convenient to that of the human body, with widths no more than about 2 1/2 feet. Any larger will make reaching across difficult and uncomfortable.
Shapes and Plantings
Square and rectangular shapes, though easy to construct, sometimes give an obtrusive look to the space. Most things in nature are not square, so creating tiers and boxes which are more diagonal, hexagonal or freeform provides a less structured, more natural feel.
By design, tiered gardens have some areas that sit higher than others. Keep this design in mind when selecting plants. Also consider how the plants will change with time. As they grow, the overall look of plants will change, which can negatively affect the look of the garden. Tiered or not, water will naturally travel downslope, so use this to its greatest advantage by planting species that require heavy water on the bottom tiers and those that prefer drier soil on upper layers.