When I was a student, living in poor housing, every house in the street had a concrete back yard, and the houses opened directly onto the street. The people two doors down had a wonderful back garden. They had all their plants in plastic pots, and had put soil between the pots (the pots were round, and there wasn't really any room between them). This had naturally gravitated to form a soil layer under the pots. It looked like a well planted garden, whereas it was just pots, that were completely removable if and when they ever moved house. They did have paths between the "beds" - just the original concrete. I can't remember if they actually had any retaining walls for the "beds", but I don't think they did. They watered their garden the same way that you normally do, because the pots all protected each other from drying out.