Castles In The Air- Jo's synonym for the girls hopes and dreams.  The castles are what the girls' wish they could do or be and they are in the air because it is not always possible to "go live in them" or make them come true.

River- In a poem that Jo writes for Beth before her sister's death, the river stands for Beth's death, and crossing it is the crossing into heaven or the afterlife.

Harvest Time- just as when plants grow ripe on the trees and fall, Harvest Time for the Marches is when all the girls are fully grown and living separate lives from their parents.  Since her teaching and guidance was so good, Mrs. March's harvest time was a happy one.