Of course, a wall doesn’t have to be physically massive to take on functional “thickness” — even a balloon-frame house can have a comfortable window seat or a wrap-around porch. And the principle of “thickening the edges” (borrowed from Christopher Alexander and company) isn’t only about walls.
What is a sidewalk restaurant but a thick zone of overlap between indoors and outdoors?
To separate two areas within Parc Citroen in Paris, a line on the ground would have done the trick. Instead, raised lawns form a thick edge. It creates a transition zone between the areas, and it invites people into that zone, and the people in that zone further “thicken” it: When you pass through, you aren’t just walking between bits of lawn; you’re walking through drifts of people.