Symbolism, Imagery, Allegory
When a novel has two women protagonists whose surnames form the two halves of the term "motte-and-bailey," you can bet there'll be some wall, tower, and battlement imagery and symbolism on the way.
If you're not familiar with early European military history, check out this fast and easy run-down on traditional motte-and-bailey fortifications. Essentially, you should be picturing a walled-in castle on a small hill, with a deep trench surrounding the base of the hill, and another wall running around a larger outer perimeter that circles the deep trench. The small hill is the motte; the area between the trench and the outer perimeter is the bailey. Easy peasy, right?
As we know from their character descriptions, Christabel LaMotte and Maud Bailey share a number of things in common: they both value their self-possession and autonomy; they're very protective of their freedom to work independently; and both of them are really reluctant to let anyone crack their defences and disrupt their work and their lives. Christabel puts it best in one of her early letters to Randolph Henry Ash, where she writes:
"Oh, Sir, you must not kindly seek to ameliorate or steal away my solitude. It is a thing we women are taught to dread—oh the terrible tower, oh the thickets round it—no companionable Nest—but a donjon.
But they have lied to us you know, in this, as in so much else. The Donjon may frown and threaten—but it keeps us very safe—within its confines we are free in a way you, who have freedom to range the world, do not need to imagine. I do not advise imagining it—but do me the justice of believing—not imputing mendacious protestation—my Solitude is my Treasure, the best thing I have." (8.49)
Christabel emphasizes her point by including figurative images of eggshells, towers, and donjons in her letters. Possession also uses these same images when it introduces us to Maud Bailey, whose office at the University of Lincoln is at the top of a tower (4.20), and whose apartment is painted and furnished in various shades of white as though she's also living inside the hard white shell of an egg (4.118).
No doubt about it, these ladies give new meaning to the notion of putting up emotional walls. For these ladies, it looks like love is gonna have to be a battlefield, and they're not just gonna give in.