I made this garden bed last year by putting cardboard on top of snow, that was on top of very long grass, and covering it with alfalfa hay and letting the rabbits eat the hay and poop in the garden:
(Bunny is Basil, he's a free range backyard bunny)
In the spring, I refreshed the hay and fenced the garden. When it came time to plant, I dug into the cardboard and there were so many worms. I planted far too densely, and let most of them sprawl. I found out that we have far too many ear wigs for that. Still got a decent crop. After the season, I pulled the plants and then just left the garden. A couple weeds came through, so I threw a bunch of grass and brush clippings from the yard and alley. I'll refresh it when I mow the back yard. This year, when I dug down, I was pleased to find out that my dirt was almost entirely worm poop, and there were still tons of worms. Hoping for a good year again.