Olalla is an engaging, well-written work of Gothic horror. We are introduced to a sympathetic narrator, a soldier recovering from a wound, and the exotic setting where he seeks to recover. . Namely, a dilapidated castle set against the Spanish mountains.
There, we meet the equally decaying family line that takes care of the home; a simpleton, amicable son named Felipe, the sensual, idiotic, and child-like mother, and lastly, the mysterious Olalla, who we don't meet until later in the story.
All the ingredients for an exciting yarn are present. Unfortunately, Stevenson takes it absolutely nowhere. It's almost as if he had half of an idea for a great short story, but finished writing without ever coming up with its conclusion. There is an interesting mystery along with a predictable love subplot, but both ultimately go nowhere.
The ending is especially unsatisfying, proving no real punchline, resolution, or point. I don't mind vagueness in a horror work that is up to a reader's interpretation, as long as we are a presented with meaningful events and a significant ending, qualities which something like "The Turn of the Screw" had, and "Olalla" doesn't.
That's unfortunate; the writing is good, the characters and concepts are intriguing, but ultimately, it's a dud.