What is main feature and the central theme of Gothic literature in "The Castle of Otranto" and "Vathek"? How are those elements represented in the texts?

Horace Walpole's The Castle of Otranto is widely considered the first Gothic novel and, as such, many of its themes and preoccupations became defining features of the genre. Its eponymous castle setting, to begin with, was reproduced in much early Gothic architecture, with the atmosphere of mystery and suspense supported by the ominous surroundings and old stone walls of an ancestral castle depicted mostly by night.
The focus on castles and grand buildings is also present in Vathek, although the types of castle and the Eastern theme are not identical to the nebulous European setting generally found in Gothic novels. As a historical novel set in the ninth-century, the time setting is also dissimilar to most Gothic, which generally explores contemporary anxieties with a contemporary setting. But the vast landscapes, hinterland waste settings, and labyrinthine castles of the novel serve as a vehicle for its supernatural themes, driven by the all-consuming and, often, Machiavellian ambition of its protagonist, Vathek.
Unlike Walpole's Manfred, who is trying to prevent an evil prophecy from coming true and is generally a force for good, Vathek is not really a hero even at the beginning of his story—he is motivated mainly by self-interest. However, we can see echoes in Vathek of Mary Shelley's Victor Frankenstein and other Gothic protagonists who, in the desire to learn all that they could learn and move beyond what should be possible for a human, toy with the supernatural—to their detriment. Vathek seeks to gain supernatural powers, but his eventual destination will be hell. Likewise, Manfred, in pursuing his goal, becomes increasingly more villainous, which causes the supernatural behavior in the castle to intensify, and, in the end, his ambitions are not realized.
So, in answering this question, it would seem logical to focus on comparing the texts' protagonists. The Castle of Otranto, with its air of mystery and suspense, its trapdoors and moving pictures, is very obviously Gothic. But what makes it most similar to Vathek is not these trappings, but the nature of the protagonist. In considering how the protagonist's desire to control the situation and pursue his ambitions is presented, it would also be pertinent to consider the respective endings. In Otranto, Manfred does repent of his devious behavior at the end of the story and survives, but he is not rewarded; his daughter, Matilda, the sacrificial female archetype of Gothic literature, is killed as a result of his actions, and in the end, Theodore is revealed to be the true prince. In Vathek, the protagonist suffers still worse a fate. Rather than becoming the supernatural, near godlike figure he had striven to become, Vathek is doomed to be tortured forever by Eblis, the demon, in a hell that resembles the most isolating wastelands of Gothic fiction.
Ultimately, then, the idea of both novels is that ruthless ambition (often found in Gothic protagonists) will not be rewarded. The difference is that, where the Christian Manfred may eventually be able to repent (albeit condemned to hell on earth), there is no specter of redemption for the non-Christian Vathek.
https://www.bbc.co.uk/news/magazine-30313775