Although the film was not released in the United Kingdom until 23 September 2005, director Hayao Miyazaki personally traveled to England in the summer of 2004 to give a private showing of the film to Diana Wynne Jones.
Symbols and metaphors are rampant in this story. The Castle is representative of Howl himself, a highly powerful yet disorganized mess, with a lot of baggage. The black gateway on the portal/dial is the way to Howl's psyche and unconscious, which he keeps himself immersed in. Calcifer, the fire, is of course the heart, quite literally. Markl could be understood as the innocent childlike part of Howl. Sophie starts by organizing things in the Castle, wins over Markl, as well as Calcifer. At the end, she is able to help Howl finally when the ring Howl gave her leads her to his innermost self, the black gateway on the dial.She visits his past, and finally is able to help him reconcile his past, his feelings(his heart) to his present state.
Sophie is established as a witch who can talk life into things in the book, while in the movie it is left up to the viewer to realize this. Some of the moments that make her powers obvious are when Sophie rescues the scarecrow and he gets deeply attached to her, as well as when her hair gives Calcifer extraordinary strength even while Howl is absent. Calcifer repeatedly singles her out, confirming that only if it is her performing the action, will he be able to live on and Howl be able accept his heart back. Sophie is able to keep Howl's heart and Calcifer alive through her inherent powers. Sophie, being a witch, also has significant power over the Witch's curse, only she doesn't know about it. In her sleep, while she is dreaming, and during deeply emotional moments, when she is upset or happy and completely in love with Howl, she turns back into her younger self, being able to undo the curse by accepting and embracing herself, giving in to her own powers.
The trivia item below may give away important plot points.
Though it's not as clear in the film, in the book Sophie is a witch herself! Each hat she makes she whispered things to or talked about the sort of person who wear it and what they would do or experience...she was unknowingly casting spells. THAT, along with Howl taking interest in her, is why the Witch of the Wastes come to the shop. Notice how the witch scoffs very particularly at the hats... The book really served as a companion to his film.