Symbolism, Imagery, Allegory
None Shall Pass
Seriously: if we saw any architecture as grim as the kind found in this book, we'd be running the other way. These mansions sound more imposing than nuclear waste storage facilities.
From the very first pages of Wuthering Heights, Lockwood is anxious to cross the threshold and enter the house, while Heathcliff seems intent on keeping him out. "Even the gate over which [Heathcliff] leant manifested no sympathizing movement […]" (1.6).
Lockwood personifies the gate, implying that, like Heathcliff, it does not want to let him in. Even Lockwood's name reflects his failure to gain access—you don't just give someone a name like Lockwood without it being pretty significant. (But since he is not one to pick up on hints, he charges in anyway.)
In his first descriptions of the house, Lockwood observes its super-unwelcoming architecture:
Happily, the architect had foresight to build it strong: the narrow windows are deeply set in the wall, the corners defended with large, jutting stones. (1.12)
Constructed in 1500, this home is clearly designed to be impenetrable. The window in the oak-paneled bed is a critical boundary in the novel, symbolizing a space of violation and violence. Even though Catherine's name is scratched on its surface, the window doesn't provide entry for her wailing ghost—thanks in large part to Lockwood's lack of sympathy. The bloodshed from Catherine's wrist "rubbed [...] to and fro" on the pane suggests that there is some serious violence involved in crossing thresholds.
Later in the novel, the young Cathy escapes Heathcliff from the same window:
She dare not try the doors, lest the dogs should raise an alarm; she visited the empty chambers, and examined their windows; and luckily, lighting on her mother's, she got easily out of its lattice, and onto the ground by means of the fir tree, close by. (28.66)
Remember that same fir-bough scratching on the window as Lockwood emerged from his nightmare?
Prison Break
There are numerous incidents in which the two houses are referred to as prisons and their inhabitants as prisoners. When domestic harmony finally returns to Wuthering Heights at the novel's end, Lockwood finds that the whole prison vibe is gone:
"I had neither to climb the gate, nor to knock it yielded to my hand [...]. Both doors and lattices were open [...] what inmates there were had stationed themselves not far from one of the windows. I could see them and hear them talk before I entered, and looked and listened in consequence, being moved thereto by a mingled sense of curiosity and envy that grew as I lingered. (32.26)
Obviously, Lockwood is still a major snoop; the problems caused by his past boundary violations don't hinder him from imposing himself yet again.
Windows Were Made to Be Broken?
Throughout the novel, characters gaze and spy through windows, open windows, and break them. Not surprisingly, the large drawing room window of Thrushcross Grange appears ample and cheery compared to windows at Wuthering Heights. Rather than being "narrow" and "deeply set," it provides accessible views out onto the garden and green valley and, conversely, into the home's interior.
When Catherine and Heathcliff venture out to spy on Edgar and Isabella, the drawing room window provides a view onto a different world—one that eventually welcomes Catherine but rejects Heathcliff. Thrown out of Thrushcross Grange (as he will be many more times), Heathcliff is left to make his observations through the glass partition:
"I resumed my station as a spy, because, if Catherine had wished to return, I intended shattering their great glass panes to a million fragments unless they let her out." (6.39)
The many symbolic meanings of windows extend even to Heathcliff's appearance, as Nelly describes his eyes as "a couple of black fiends, so deeply buried, who never open their windows boldly" (7.42). Again, windows prevent rather than provide access.