Yester-day I listened to the Byrds' Younger than Yesterday, and I noticed a small thing about "Mind Gardens." The "walls all down" part of the line "I tore the walls all down" is sung to a descending phrase (E D C#), so there's a musical representation of the walls' coming down.
Just for the sake of making this post a bit more than just "hey, there's significance to this descending phrase," I was going to note the origin of the phrase "the slings and arrows of outrageous fortune," which occurs earlier in the song, but in doing so, I actually found something I hadn't realized before. I've known for years that this is a phrase from Shakespeare's Hamlet. It's actually in Hamlet's "To be or not to be" soliloquy. He asks himself "Whether 'tis nobler in the mind to suffer / The slings and arrows of outrageous fortune / Or to take arms against a sea of troubles, / And by opposing end them." (III.i.65-68). But I hadn't realized that both Hamlet and David Crosby's song are talking about minds. Hamlet specifically mentions "in the mind," where Crosby seems to use a garden as a metaphor for a mind.