Here's my version of it as a step by step:
The steps are:
1. Create a selection that encompasses the region that you want the earthworks in. Here I got all your outer wall outline and expanded by about 100px to get the outer edge. For the inner edge I did the same and expanded the selection by about 10px.
2. On a new layer fill the region outside the eathworks with 50% grey. Fill the inside region with black (this will ensure you have a steeper edge inside the earthworks than outside. In gimp I'd do this by creating a new selection that runs down the middle of your earthworks (so outline of walls +55px in this case) and fill inside with black, outside with grey. Then take your earthworks selection and use the gradient tool set to shaped spherical, and the gradient type to foreground to transparent with a foreground colour of white. Apply the gradient to your selection (click and drag) and you should get something like 2 for this layer.
3. Create a new layer and fill it with 50% grey. Go to Filters->Map->Bump Map. In the Bump map drop down, select the layer that looks like 2 (the black white and grey image). Fiddle with depth and angle of light until you get something you like. Apply the filter. Now this layer probably has jagged artefacts on it, so it's probably worth applying a 5-10px Gaussian blur on it to smooth those out.
4. Change the blend mode of this layer to overlay or soft light and make sure it's over your grass layer. Fiddle with the opacity until you're happy with it. You can also duplicate the layer and set the blend mode to darken only to bump up the shadows. Make sure the layer that I have as 2 in the walk through is turned off - you won't need that again.
You can then go on to add erosion effect to your overlay layer, but that should give you the basic structure of your berm. Remember that your walls are going to cast shadows too - so you'll need to alter your overlay layer to show that as you go forward.
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