Hillsides can be daunting because concrete or block retaining walls are expensive and often, in time, they will fail or tip out. The land knows where it wants to be and I would retain it where it is rather than trying to impose a new shape upon it. Here is a fairly inexpensive plan that will make soil retention and access a lot easier. The materials are very simple: linseed oiled redwood planks or pressure treated landscaping lumber cut into three or four foot lengths, and 5/8” rebar, cut into lengths varying from three to five feet long.
Firstly, if the soil has a clay content you will want to be very careful to ensure that the planks are sturdy enough. Use 2 x 10 or 2 x 12 lumber. Railway ties are heavy and should not be more than two rows high.
If your hillside is gentle and your soil is sandy or loam, use 2 x 8s. Start at the bottom and work with the lay of the land, so that the planks follow the curves. Set a plank on the hillside, angled just a little bit in toward the hill and settle it into the soil by tapping it down. Place two three foot long pieces of rebar in front of the plank to hold it in place and hammer it into the ground with a small sledge hammer. Hammer down on the plank to ensure that it is solid and then tap the rebar down. Backfill the plank and then you can stand on this to work above. Offset the planks so that water cannot gain a long path through the gaps. Carefully wedge in a rock at the ends of the planks to stabilize the soil. Use a piece of rebar to hold it if it looks like it could roll out. Work your way horizontally and up the hill.
For steeper grades, use 2 x 12s and four or five foot long rebar and step on the layers as you work up the grade, to ensure that all is stable.
When the retention is complete then you may put in drip irrigation lines and plants. For steep slopes use lightweight deep rooting plants such as Salvias, (leucophylla point sal and bee's bliss are good ones), low growing Coyote bush, Lantana (not the invasive pink one), Epilobiums, prostrate rosemary, smaller Aloes, Sempiveriums, Parry’s agave, and other smaller succulents. If you can find Dalea greggii it is an excellent ground cover for hillsides. Avoid invasive or prickly plants to keep maintenance simple. When all is done, add a layer of mulch to protect the soil and slow water penetration.
Graptopetalum paraquayense is a very useful succulent for hillside planting. It is lightweight, fast spreading, easy to remove and very drought tolerant. Here it is seen tumbling down over a wall in the harshest of locations, facing south west on a dry slope. Note the mimulus on the top right. Eventually it will become large and replace the graptopetalum.
Another way of treating a hillside is to use broken concrete to construct terracing. This method of retention works well with gentle slopes and will add a semi-formal appearance. Concrete is weighty and cannot be used on steep grades.
If you look at the drawing above you will see that the concrete is sloped back into the hill. This will ensure that should there ever be displacement, earthquake, unusually heavy rain etc, the concrete terraces will fall back into the hillside and not give way. Be sure and compact the soil as you build the terraces. This form of retention will last a lifetime or more.