On sloping land, erosion caused by run off can kill the soil. It also creates uneven distribution of rainwater over the land, which naturally does less for plant life than it could if it were evenly distributed.
Courtyards Which Live --> HERE Garden Growing Wild --> HERE HERE HERE --> Building Edge HERE --> Greenhouse
On all land which slopes - in fields, in parks, in public gardens, even in the private gardens around a house - make a system of terraces and bunds which follow the contour lines. Make them by building low walls along the contour lines, and then backfilling them with earth to form the terraces.
Try Lineup Viewer