It remains only for Sophie to brandish her stick and yell “Stop!” before the Castle runs right over her — or at least the reader thinks it might run right over her — and obediently, a short distance uphill from her, it stops. 
Up close, the castle is tall, crooked, and top-heavy. Newgrange-like, it appears to be faced with closely-set stones, but they are not the white quartz of the glass castle; instead they are misshapen lumps of coal. They exude a deathly chill, but Sophie’s not focused on that. She just wants to get to the warm fiery place inside that’s making all the smoke pouring out of the turrets.
Despite the Castle’s — or its owner’s — best efforts, Sophie doesn’t imagine some hellish soul-searing inferno within; she’s picturing “chairs and firesides.”  This could be owing some magical sight of her own, penetrating the outward appearance of things. Or some other intelligent will might be working upon her perceptions. In any event, she thinks she’s got the thing eating out of her hand, until she walks up to the big black door in the blank wall before her and tries to knock.
There’s a force-field in place, impervious to her prodding finger, her stick, and her order to “Open up!”
Looking for another door, she tries going around to the castle wall to her left, but she can’t even get past the corner.
At this, Sophie said a word she had learned from Martha, that neither old ladies nor young girls are supposed to know, and stumped uphill and anticlockwise to the castle’s righthand corner.
The castle forces her to go widdershins; in other words, in the direction counter to that of the sun’s motion: another way of warning anyone off who gets too close.
[I am embarrassed to admit to the Blinding Flash of the Obvious that befell me while looking into this matter: I had never quite put it together that a clock face is arrayed like a compass rose, with north at the top, east at the right, south at the bottom, and west at the left. As the sun moves from east to west the numbers get bigger and the hour gets later, until after two rounds it resets at midnight, bumping the date one day forward. Sheesh. Knock me over with a feather.]
Also, I very much doubt that Sophie’s prim younger self would have said whatever that word was out loud. [I detect, too, a faint, Austenian hint that Fanny and Martha are maybe a wee bit less refined than was Sophie and Lettie’s mum.] Growing old does liberate one’s tongue. At the same time it turns you salty and cynical about some people and things. Many people and things.
There’s another door on the right side, and another force field. In describing the castle’s outward attributes, I begin to suspect that Jones is subtly and cheekily describing its owner as well:
Black smoke blew down from the battlements in clouds. Sophie coughed. Now she was angry. She was old, frail, chilly, and aching all over, night was coming on and the castle just sat and blew smoke at her. 
Yes, guys do act that way, sometimes.