A curved retaining wall is easily made in the 3D Custom Workshop. Select 2D Top View. With the Polygon tool draw the outline of the wall - as if you were viewing the wall from the top. In the Options menu select Point Selection. Select all points except those at the ends of the wall. Apply Curve Smoothing (Options menu) to those points. By adjusting the amount of Curve Tension and the position of the points you should be able to get the desired shape. Click the Fly-Around (helicopter) to return to a 3D view. Return to Object Selection from Point Selection. Use the Extrude tool to convert the 2D shape into a 3D wall. Select either the Front or Side grid. Double Click the wall to get the Object Size function. Select Object Size in the dialog box. Change the value in the Height field to the desired height of the wall. In the Size From part of the dialog box select either Lower option. Click OK. Closing the Workshop will give you the option to add the wall to your plan. As Patricia suggests, make sure the real life wall is safe and meets code.

The Topo Designer, one of the Power Tools in AS 4000 is the principal way to adjust the topography (pile up dirt).