Purpose
A brick wall on uneven ground can serve several functions. Besides adding to your yard's appearance, it can help section off different portions of the yard, such as flowerbeds, a pool area and a turfgrass lawn. On uneven ground, a brick wall can serve a retaining purpose, preventing the ground from eroding or protecting your home and yard from a higher grade at the edge of your property that if unobstructed could spill over into your yard.
Preparation
Whether a brick wall is build on level ground or uneven ground, it requires a trench to lay a foundation that eventually is hidden underground. For uneven ground, the trench is not a consistent depth. Instead, the trench needs to consist of a number of different depths, each the same distance below the planned top of the wall. This means that the trench and the wall design both have a step pattern that is roughly parallel to the grade of the yard's slope. The foundation can consist of gravel or of bricks that are wider than the planned width of the wall.
Masonry
Constructing a brick wall using an uneven trench is similar to the masonry process for a level brick wall. You must mix the mortar properly and work quickly to apply it before it dries. A level is useful for checking the plumb, or vertical, angle and the level, or top slope, of each section of the wall. You also need a measuring tape or yardstick to measure the gauge, or height, of each corner above the base of the trench.
Finishing
Once your brick masonry is done, you'll have a brick wall that traverses an uneven portion of your yard, with an uneven top that steps down, one or more bricks at a time, to maintain a consistent height. To make the top of the wall smooth, you can apply a layer of mortar or plaster that covers the top bricks. You also need to fill in around the foundation by replacing some of the soil you removed while digging the trench. Cover the foundation only after the entire wall has had time to dry.