"Who knows what evil lurks in the hearts of men? The Shadow knows!" Algernon Blackwood beat Walter Gibson to the punch with this meme, albeit nothing actually happened with Blackwood's shadow.
What is terror? Is it a physical calamity the evokes the stultifying, and sometimes maligned emotion? No, it the portentous foreboding that something might or could happen, of something just around the corner-or right behind you, dredged up from the accumulated recesses of your mind. It is the fear of the unknown and the expression of such that the mind tries to form. And that is where Algernon Blackwood takes you in this story.
A distraught woman invites two friends to spend time with her at the towers, a mammoth estate that is too big and too much for her to bear alone. Her late husband made the place his own, in that everything in it and about it was a reflection of him, not her. Gone, but not forgotten, the old maid on the property was there to keep his ghost alive.
The two friends, brother and sister, soon begin to feel the uneasiness of their host. The late fire and brimstone husband had unwittingly tormented his wife and induced a kind of psychosis, that spread like a blanket over the two guests. But, as reiterated numerous times in the book, nothing happened. But that isn't exactly true. Plenty happened, but if you were expecting an action-packed horror story, you will be disappointed.
Without going over the tale top to bottom, the story reads like an allegory, Frances and Bill, the guests, melding into a single mind with Mabel, the host. Frances was half-way between sanity and Mabel's condition, while Bill was the level headed one, although even he felt uncomfortable to the point where he was unable to relax at all.
Bill is the storyteller here, but Blackwood explores the fears of all three characters, a dynamic of one mind unsettled with fears quite possibly adopted in childhood (even alluded to in the story), with the house itself seeming being that part of the mind serving as a prison for that part of the mind that is looking for an escape.
In the end, a casting off of irrational fears and beliefs can work wonders seems to be the message. Unfortunately, many of us never achieve that; we can only assume that it happened here. But we'll never know for sure.