Thirteen-year-old Eldora has moved to San Francisco with her aunt and uncle. In her letters to Cousin Sallie, she interweaves her story with that of the new city. The corrugated castle in the title is Eldora’s new home, an iron structure built in two days from a kit. While the heart of the early letters are an anticipated meeting with her long-lost mother and the possibility that she is not an orphan after all, the story eventually incorporates a newspaper column about San Francisco and various letters, including those from her new friend Luke. Eldora’s letters in this epistolary novel are a contrivance, relating much history but never sounding like real letters, and readers—beyond history buffs—may find the story slow going. However, Eldora is a likable character and readers will enjoy seeing her find her way in a new city in the process of creating a unique identity. (glossary) (Fiction. 10-14)