Sissinghurst is a place in Kent, in the south of England, known for a house with a tower — people call it a castle — and for the gardens around it. The writer Vita Sackville-West and her husband made those gardens last century, room by room, like outdoor rooms with walls of plants. Garden lovers know the White Garden especially. The word is a proper name: it is this one estate, not a kind of castle. You will see it in travel pages, garden books, and English heritage lists. Though actually the “castle” is more a grand old house with a surviving tower than a fortress with knights; the gardens are why most visitors come.