Some titles are tricky, Shmoopsters—and goodness knows, plenty of characters in this book are too—but Westmark as a title is about as simple as they come: it's named after it's setting.
Westmark is the name of the kingdom in which this whole adventure takes place. While Theo and Mickle are the main characters in the book, the story isn't just about them—it's also about the evolution of a kingdom and its people, from a state virtually ruled by tyrannical Chief Minister Cabbarus to its rightful restoration to a just monarchy under King Augustine and his heir, Princess Augusta (formerly known as Mickle).
Just as we see Theo and his cohorts mature and realize the complexities of life, Westmark the nation goes on a similar journey. It travels from rule under a despot to a decent, non-repressive land once again—all thanks to Theo and Mickle, of course.