This song is about a woman who has apparently married a very well-to-do man. She may be, to him at least, a trophy wife, and he has set her a up in a mansion on a hill with a fence around it to protect her from the world. There is a certain melancholy to the song, and we are given to believe that the mansion is very much a prison to her, but rather than lose the privilege and prestige inherent in her marriage, she chooses to remain in that prison.
Suggestion credit: Peter - Newark, NY
The "hissing of summer lawns" refers to the sprinklers that water the lawns of the upper class in Bel Air, a rich enclave of Los Angeles. Mitchell bought a house there in 1974; the inside cover of the album contains a picture of her in her pool.