I guess I just got lucky. When we bought our house, these wedge-shaped blocks were already on the property. It was quite an ordeal to haul all 70 of them from the back property line but well worth the effort. After counting, measuring and laying the blocks, I tilled up the 4 inches or so of topsoil at the bottom. I then placed a soaker hose about 2 inches into the natural topsoil. The hose was snaked up through the blocks so I would have easy access for watering. The small black object projecting from the far left side of the blocks is the end of the hose. Next, I used heavy duty plastic sheeting around the inside of all the blocks to prevent them from pulling the moisture from the soil. Then, I ordered 18 Cubic Yards of a mixture of 60% topsoil, 20% compost and 20% coarse sand from a local landscaping company. This configuration required about 10CY so I used some more of the mixture for containers and stockpiled the rest. I have been gardening in this location for two years now and it beats any other garden I have had in the past 40 years or so by a large margin. I have access from the outside of the blocks as well as from stepping stones running through the middle. I am a fairly large guy so I can reach any part of the garden without too much problem. I assume that you are planning something a little simpler and less permanent but maybe this will give you some ideas.
mater