Well, it depends on how high you want to go. Different sizes of "bricks" or "stones" for the height and weight pushing against it.

I had to build umpteen retaining walls and ended up choosing Cottage Stones from Mutual Materials via Home Depot. Best price, best delivery. Guy said it's even cheaper than pouring concrete, and is idiot-proof, and do-able at 30 lbs per "stone." But my walls are 2' or less above ground.

I did the first few feet the way they say with the gravel and drainage but that actually didn't work in my yard. Best way was just to tamp down the clay and make the first level perfectly level in a 5" dug tamped trough, and then fill in every triangle with that clay as I built.

Good luck! They take patience and that first row must be perfectly level but then you have years of very handsome useful walls. Only ivy and euonymous fortunei grow UP these walls.

What look are you aiming for? How high is this wall going to be? Over a certain height, you will need permits and perhaps engineering.

I built a 18 inch high wall using cobble sized granite rocks dry fit and it's held up for a year now. I love the look of it but it's a lot of lifting.

if you keep your height under 3 feet, maybe push it to 4 feet or so, your options are much better. you won't really need "engineering". or i guess you could say, all the engineering you need will be gravity pushing rock downwards. and dry stacking works well at this height, so you don't need as much material or skill.

can you pile things on top of one another? YOU GOTS THE MAD SKILLZ FOR DRY STACKIN

i like scrappy walls, so a lot of craigslist "free" and broken up concrete sidewalks will get you there for, uh, FREE.

i've built those cottage stone walls. they don't look so horrible once the moss grows on them, but the main point here is the stunninly ugly effect they bring onto this world. OK, i exaggerate. point is: not pretty. but, as pointed out above they are cheap and easy and you can push the 3-4 foot barrier with them safely.

What's it supposed to do there on your site? How much weight is it supposed to hold back, any seepage involved?

Ideally, whenever possible a garden area is graded to flow gradually down from one level to the next instead of being abruptly cleft or bisected by a wall.

well said everyone.I would also consider how sunny,wet , shady will it be? Do you have moss there now? do you need to add proper drainage.You are asking very good question so consider above.Also like everyone said what are you going to grow there?

It is impossible to properly answer your question without more information :-) We need to know the proposed height of the wall and what it is retaining (i.e., a slope?). Portland building codes require that walls heigher than 4' go through a permitting process (which requires structural drawings/engineering) or walls of any height if they support a surcharge. A surcharge is any load beyond the normal weight of level backfill = a yard with a slope of 2:1 or greater.

One of my favorite articles on retaining walls starts out "any moron can build a retaining wall......and many have" :-) While simple, low retaining walls are very much a DYI project, you still need to know some basic principles of retaining wall construction - creating a proper base, footer or deadmen requirements, stepping back the wall, drainage issues/what to use as backfill. Remember that water adds weight and "pushes" outward....as with many landscaping soil issues, addressing proper drainage is a critical issue. Attempting to build retaining walls without proper research into recommended construction methods will very often lead to failure of the wall down the road. It is worth the time and effort and the expense of doing it right the first time.

There is no valid reason not to build a wall, if that is what you want and what will work for your property - grading a slope is one solution but not the only one nor necessarily the 'ideal' one.....it depends on the specific situation. FWIW, the interlocking concrete blocks that everyone abhors are very inexpensive and very efficient..... for walls upto 3' high. They are also extremely easy to build with, another reason that makes them so popular. And what you are going to plant behind it/on it has very little bearing on the type of wall or the materials used, unless you are planning on planting a tree with an aggressive root system at the crest of the wall.

Information is lacking on your conditions and circumstances. Why do you feel the need for a wall? I know, you have a slope. Is there anyway you can moderate the slope and live without a wall? bboy brought that up. Walls are expensive. Changing the slope is less expensive.
Are you looking to make everything level? So it's 'usable'?
I like walls. I've made many in 30 years of Landscaping for a living. They can make a real statement in the garden, but they have to have a reason. No air walls.
I'd like to see a picture before I go any further. Give us a reason.
Mike....or a picture would be nice.