The keyhole garden method is described in the article below which includes instructions showing how to make one yourself.

Keyhole Garden Tutorial

Here?s another small space garden method to try. Dr. Deb Tolman is the original creator of this concept of rolling a garden bed and compost into one. It?s based on the lasagna garden method where layers of recyclable materials are used as well as being water conservative.
Follow these simple steps to create your own and you?ll have a veggie bed that is a pretty as it is Eco sensitive.

What is a Keyhole Garden?

A Keyhole garden is a raised bed that uses minimal space, recycled materials and combines a compost system and garden bed into one.
It?s a circular shape with a slice taken out to allow for easy access to the compost basket in the center creating the keyhole shape. Build the sides out of wood, cinder blocks or bricks. Any type of material could be used that would allow you to build up the sides.
 

 
 
 
 
 
 
 
 
Use two sticks with a 3? string tied between them to measure out your shape. Place one in the center and draw a line turning around the center point. Build your sides with your chosen material up 2 feet. We used 48 cinder blocks in our example.
 

 
Similar to the lasagna style, a layer of rocks, twigs, branches and even old broken pots can be placed in the bottom to allow for drainage. Add layers of cardboard and the newsprint between compost and then fill the top half with straight soil/compost mix.

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