A retaining wall is a special kind of wall, which has to not only support itself but has to actively hold back pressure from the material behind it. Here are some of the most important things to remember when you're building a retaining wall.
The rocks you use, however you use them, have to resist the pressure being applied by the weight behind the wall. The weight of the wall will press down on the base material you are building on, which may make the material shift, compromising the structural integrity of your work. Take the time to build a solid base if needed. Water collection behind walls is unacceptable. The increased weight of wet gravel or earth, and the high risk of heaving due to frost, can threaten the wall. Thus drainage is important. Any multi-level tiers should be planned out to ensure that the top tiers do not add to the pressure behind the underlying tiers.
Finally, your wall should be a part of your landscape that marries well with your home, your tastes, and your lifestyle.
Here is an example of a reinforced stone wall. Notice the footing is 3' deep and 30" wide with re bar. This wall was over 6' high.
This is a great example of a cross section of a stone wall we built in Westwood. Notice how thick the wall needed to be to hold back the hill.