To make the corners of the garden sturdy I cut the timbers to interlock. Then I drilled a hole all the way through and hammered in a 36″ rebar pin. I did it the same way for the first half and it's worked out pretty well.
You can see again, I'm using newspaper to help keep the grass from growing up to the garden surface.
Originally I left the pins sticking out about 2 feet. I was using it to hold up my fencing material. I'm not doing that now so I hammered the pins in almost all of the way. I am going to leave a few inches in case I decide to add another level or two.