Gardenning ih dry difficult conditions

The cross section through my garden starts with a 2 metre wide paved strip at house floor level. Next is a retaining wall 1.2 metres high. Then another paved strip 2 metrs wide at this higher level and then another retaining wall about 1.00 metre high . The ground then rises at 30-40 degrees to the rear fence. The overall depth of the garden is 9 metres. The ground is stoney sand and is in the shade for most of the day, There are a few established shrubs, small trees. Water runs quickly off the surface of the remaining bare patches . Has any one got any ideas?