"The keyhole garden was developed in Lesotho by the Consortium for Southern Africa Food Security Emergency (C-SAFE), based upon a design that originated with CARE in Zimbabwe."
"......lettuce, kale, and spinach; herbs; and root crops such as onions, garlic, carrots, and beets. Keyhole gardens are ideal for intensive planting, a technique in which plants are placed close together to maximize production."
They help with the solution of growing produce in areas of arid land or land with poor soil and are particularly useful as a kitchen garden due to their flexibility in what can be grown, the use of kitchen waste/grey water and that they can be highly productive. One has recently been made in the garden of Mrs M's office as a tool to show others what can be done. They do work. en.wikipedia.org/wiki/Keyhole_garden
Just thought I'd throw this in there for you to have a look at if you've never come across them. Several short videos may give you a better idea -