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The stem wall must be level to support a structure. Place a level on top of the wall and adjust the bricks if necessary. Check the wall in multiple locations to confirm that it's level across its whole perimeter. The stem wall only forms a part of your foundation. If you intend on building a structure, you need additional foundation. This usually requires pouring cement into the section that the stem wall surrounds. Pouring this much cement is a big job. Consider contacting a contractor to do this step for you. You can also build a foundation with large rocks or bricks. This is only for a smaller structure. The dry stack technique is only intended for walls less than 8ft (2.4m). Taller walls need extra reinforcement like steel rebar and cement. This is a different and much larger project that requires a professional contractor.
Check that your wall is level when it's complete. Continue building your foundation. Use extra reinforcement for walls taller than 8ft (2.4m).