Here is how I make my soft hills. Hope it helps.

1. First place your structures that you want on top of the hill.

2. Now with the shadow texture and a lower opacity make the shadow of the hill.

3. Make a light line around the front of the hill to indicate the light side of the hill.

4. Place the remaining stamps

5. Place your light source.

6. Project shadows of stamps away from the light source

7. Any stamps in the hill's shadow needs to have it's brightness reduced.