I can't argue with that assessment, but I think SPEC can be classified a bit more.
When we think of Gothic fiction and Sherlock Holmes, The Hound of the Baskervilles is probably the first thing that comes to mind. But I think SPEC got there first.
Gothic fiction (a broader term than Gothic horror) requires three big components: a monstrous or threatening villain, an old estate or castle setting, and a mood of dread and danger. And story delivers all three.
Grimesby Roylott is one of the best villains in the Canon. And that's because he's one step away from being a Gothic monster himself. From a long line of brutes ("Violence of temper approaching to mania has been hereditary in the men of the family.") and part of a family that almost seems doomed to fail (four successive heirs were of a dissolute and wasteful disposition, and the family ruin was eventually completed by a gambler), Roylott almost qualifies as being from a cursed lineage. And by having mastery over dangerous and threatening animals such as a cheetah, baboon, and poisonous snake, Roylott keeps those in his household and surrounding scared of him.
And just listen to some of the phrases that Watson uses to describe this doctor when he visits Baker Street:
"a peculiar measure" of clothing
"a hunting-crop swinging in his hand"
"his hat actually brush[ed] the cross bar of the doorway and his breadth seemed to span it from side to side"
"A large face, seared with a thousand wrinkles"
"marked with every evil passion"
"thin fleshless nose"
"a fierce old bird of prey"
Combine those phrases with Roylott's show of power bending a steel poker with "his huge brown hands" and later his "hoarse roar" as he bellows at boy driving a trap and you have a character that isn't too far away from Frankenstein's monster. In fact, Roylott's last act upon this earth is to release a "horrible cry which ... swelled up louder and louder, a horse yell of pain and fear and anger all mingled in the one dreadful shriek. They say that away down in the village, and even in the distant parsonage, that cry raised the sleepers from their beds."
A true monster.
While many Gothic villains are equated with castles, the forlorn manor house can be just as intimidating of a setting. Helen Stoner and her late sister, Julia, are essentially prisoners in Roylott's "two-hundred-year-old house, which is itself crushed under a heavy mortgage." Helen takes pains to describe this manor to Holmes as to emphasize the character of the place where haunting whistles foretell doom. But Watson's description when he finally sees Stoke Moran for the first time really elevates this setting to Gothic proportions:
"The building was of grey, lichen-blotched stone, with a high central portion and two curving wings, like the claws of a crab, thrown out on each side. In one of these wings the windows were broken and blocked with wooden boards, while the roof was partly caved in, a picture of ruin. The central portion was in little better repair, but the right-hand block was comparatively modern, and the blinds in the windows, with the blue smoke curling up from the chimneys, showed that this was where the family resided. Some scaffolding had been erected against the end wall, and the stone-work had been broken into, but there were no signs of any workmen at the moment of our visit. Holmes walked slowly up and down the ill-trimmed lawn and examined with deep attention the outsides of the windows."
"Curving wings," "claws of a crab thrown out," broken windows, a roof partly caved in, "ill-trimmed lawn"... Is this a place you'd want to hang out? You know the kids in the neighborhood think this house is haunted!
And what is it like living in such a dreary setting? "We no feeling of security unless our doors were locked," testifies Miss Stoner. And she describes the windows as being "[V]ery small ones. Too narrow for any one to pass through." Once those bedroom doors are locked, the step daughters are essentially trapped! And while Helen and Julia Stoner are locked away in their rooms, a cheetah and baboon wander the grounds freely. They never said so, but it is hard to miss the juxtaposition of freedom vs. being caged between the people and animals under Grimesby Roylott's charge.
Once Holmes and Watson have started their silent vigil inside or Stoke Moran, they are in literal darkness. As if to amp up the effect that this house has on the reader, Holmes tells Watson, "Do not go asleep; your very life may depend upon it."
If Stoke Moran isn't a house that invokes terror, I don't know what is.
It is Fear, Mr. Holmes. It is Terror.
The villain and the setting of "The Speckled Band" are enough to make this a top-tier Sherlockian adventure, but the mood that is set throughout this tale is what pushes this story from "a ripping good yarn" into the realm of Gothic fiction.
In the opening paragraph, Watson sets the scene by telling his readers that Holmes "refused to associate himself with any investigation which did not tend towards the unusual, and even the fantastic." This is hardly the typical introduction that tells us that Holmes will do something impressive or this case will be a difficult one. No. This case will be unusual and fantastic.
In a subtle nod to the dark mood, Helen Stoner tells her story of woe while she is dressed in black. She appeals to Holmes as if he has superhuman powers: "I have heard, Mr. Holmes, that you can see deeply into the manifold wickedness of the human heart." While she can't say specifically what terrifies her, Miss Stoner admits that her "fears are so vague." All she knows is that her stepfather is somehow involved, saying later that "He is so cunning that I never know when I am safe from him."
Describing the death of her sister, Helen states that Julia's face was "blanched with terror" and shrieked in a voice which could not be forgotten. And when did all of this take place?
"It was a wild night. The wind was howling outside, and the rain was beating and splashing against the windows. Suddenly, amidst all the hubbub of the gale, there burst forth the wild scream of a terrifies woman."
She may as well have started her story off with "It was a dark and stormy night...."
After Miss Stoner has left, Holmes himself adds to the Gothic atmosphere by invoking the Evil One by yelling out "But what, in the name of the devil!" when Grimesby Roylott arrives to threaten our heroes. Later on, Holmes will predict terror when he tells Watson that their vigil at Stoke Moran will result in "horrors enough before the night is over." As the denouement of the story comes, Watson is blinded, essentially blinding the reader as well. All we know is that Holmes's "face was deadly pale and filled with horror and loathing." Not even during the climax of this terror are we to know what is preying upon the Stoner sisters.