Martha rocked on her stool, grinning all over Lettie's face, twirling her thumbs in a happy pink whirl. "I want to get married and have ten children."
"You're not old enough!" said Sophie.
"Not quite, Martha agreed. "But you can see I've got to start quite soon in order to fit ten children in. And this way gives me time to wait and see if the person I want likes me for being me. The spell's going to wear off gradually, and I shall get more and more like myself, you see." (2.10-12)
Sophie has a really rigid view of home life according to the fairytale rules of Ingary. Her upbringing makes Sophie believe that since Martha is the youngest daughter of three, it's Martha's job to go off and have adventures. And Martha's not interested in adventures: she wants a husband and ten children. There is an interesting reversal here that, according to the clichés of fairytales, Martha's desire to settle down and have a family is actually more transgressive and rebellious than becoming a witch's apprentice would be.
Night was coming on and the castle just sat and blew smoke at her. "I'll speak to Howl about this!" [Sophie] said, and set off fiercely to the next corner. There was no barrier there—evidently you had to go round the castle anticlockwise—but there, a bit sideways in the next wall, was a third door. This one was much smaller and shabbier.
"The back door at last!" Sophie said. (3.8-9)
As Sophie first approaches the moving castle, there is already a difference between the castle's reputation—what Howl wants people to believe about it—and what is actually true. Sophie first tries the two imposing, black fake doors, but the only real door that she can go in and out of is the small back door. And in fact, that humbleness of the real castle is really consistent with the ordinary home life that Sophie enjoys there (give or take a fire demon or two).
There were a number of probably wizardly things hanging from the beams—strings of onions, bunches of herbs, and bundles of strange roots. There were also definitely wizardly things, like leather books, crooked bottles, and an old, brown, grinning human skull. On the other side of the boy was a fireplace with a small fire burning in the grate. It was a much smaller fire than all the smoke outside suggested, but then this was obviously only a back room in the castle. Much more important to Sophie, this fire had reached the glowing rosy stage, with little blue flames dancing on the logs, and placed beside it in the warmest position was a low chair with a cushion on it. (3.17)
The wizardly things in Howl's moving castle make it obvious that this is a wizard's home—our kitchens are woefully lacking in "leather books, crooked bottles, and an old, brown, grinning human skull." But the important point is not what's different in this description of a kitchen, but what's the same: this is clearly the warm heart of the castle. The rosy fire, the low chair next to it—this is a warm, comforting space for Sophie to build up her confidence.
[The kitchen] was quite a small room, with heavy black beams in the ceiling. By daylight it was amazingly dirty. The stones of the floor were stained and greasy, ash was piled within the fender, and cobwebs hung in dusty droops from the beams. There was a layer of dust on the skull. Sophie absently wiped it off as she went to peer into the sink beside the work-bench. She shuddered at the pink-and-gray slime in it and the white slime dripping from the pump above it. Howl obviously did not care what squalor his servants lived in. (4.5)
While the kitchen/main room of the castle is really homey and welcoming compared to, say, the hat shop that Sophie has just fled, it has still been neglected. Everything is filthy, grimy, and generally covered with slime—it needs Sophie and her care, which she's ready to give (in her own rough way).
There's a big difference between the work that Sophie chooses to do here and the work that she feels required to do at the hat shop. Where before her endless work frustrated her, in the moving castle her work gives her life a sense of meaning and structure because she makes a choice to do it.
Sophie was feeling decidedly queer again when they reached the Palace. Its many golden domes dazzled her. The way to the front entrance was up a huge flight of steps, with a soldier in scarlet standing guard every six steps. The poor boys must have been near fainting in the heat, Sophie thought as she puffed her way dizzily up past them. At the top of the steps were archways, halls, corridors, lobbies, one after another. Sophie lost count of how many. At every archway a splendidly dressed person wearing white gloves—still somehow white in spite of the heat—inquired their business and then led them on to the next personage in the next archway. (13.1)
Sophie's home country of Ingary is like any fairytale land, and fairytales aren't really focused on realistic systems of government or anything like that. Of course there's a King, and of course he lives in a fabulous palace—that's what happens in fairytales. But this home is the one place in the book that doesn't really say anything about its occupant's personality. It's an official residence rather than a home. When Sophie meets the King, she also has this sense of doubled vision: that he is a private person and an official King in the same body.
"Sophie, if we were to take that hat shop, what would we sell?"
Sophie found she had had enough of hats to last a lifetime. "Not hats," she said. "You can buy the shop, but not the business, you know."
"Apply your fiendish mind to the matter," said Howl. "Or even think, if you know how." And he marched away upstairs again.
Five minutes later, down he came again. "Sophie, have you any preferences about the other entrances? Where would you like us to live?"
Sophie instantly found her mind going to Mrs. Fairfax's house. "I'd like a nice house with lots of flowers," she said.
"I see," croaked Howl and marched away again. (15.13-18)
Even though we've got about six chapters to go before Howl and Sophie actually declare their feelings, we can already see signs that Howl thinks a lot about Sophie. When he is trying to imagine what their new home space would be like, he designs it with Sophie in mind—even though he presents his plans in the least romantic, most casual way possible.
Sophie opened the door and the cat crawled inside. The cat crawled to the hearth, where Calcifer was down to the merest blue flicker, and, with an effort, got its front paws up onto the chair seat. There it grew rather slowly into Howl, bent double. (16.42)
The moving castle is interesting as a home. It's as changeable as Howl himself, what with its multiple entrances, its windows onto many places, and the fact that it, well, moves. But when Howl gets back from fighting the Witch, the moving castle still seems like an absolutely stable place of refuge—like the only place that Howl can feel really safe.
The room rocked and settled. For a few instants, while the smoke still hung everywhere, Sophie saw to her amazement the well-known outlines of the parlor in the house where she had been born. She knew it even though its floor was bare boards and there were no pictures on the walls. The castle room seemed to wriggle itself into place inside the parlor, pushing it out there, pulling it in there, bringing the ceiling down to match its own beamed ceiling, until the two melted together and became the castle room again, except perhaps it was now a bit higher and squarer than it had been. (17.15)
There is a lot of poetic justice to the fact that Sophie returns to the hat shop where she started, the same hat shop from which the Witch's curse ironically liberated her. But this Sophie is a very different person from the one who was almost too shy to leave the shop to visit her sisters at the start of the book. She has more confidence now to face the place where she came from, and to make peace with her family.
They went through and out into the yard Sophie had known all her life. It was only half the size now, because Howl's yard from the moving castle took up one side of it. Sophie looked up beyond the brick walls of Howl's yard to her own old house. It looked rather odd because of the new window in it that belonged to Howl's bedroom, and it made Sophie feel odder still when she realized that Howl's window did not look out onto the things she saw now. She could see the window of her own old bedroom, up above the shop. That made her feel odd too, because there did not seem to be any way to get up into it now. (17.44)
The fact that Sophie can see the window of her old bedroom but can't actually get to it anymore suggests symbolically that she can't just fit back into her old life. Howl, Michael, and Calcifer have changed Sophie in the same way that the moving castle has altered the hat shop and its yard: you can still see the outlines of Sophie's old ways, but her personality is quite different now—or at least, the ways that she is willing to show that personality have really changed.