The backyard used to be a wreck. Once I started pulling up the rotting deck that sat in the middle and the crumbling cinder-block retaining walls around it, I unearthed piles of broken concrete and discarded bricks. But my wife, Vivian, and I had big plans for the forlorn patch of dirt when we bought our Brooklyn brownstone three years ago: We'd make it into a little urban oasis, with a patio surrounded by plants and a cherry tree for privacy and shade. It would be a place to entertain and chill out after work. Here's what we did.