As other reviewers have pointed out, this novel is compared, somewhat erroneously, to Tolkein's Lord of the Rings, because both fall loosely under the heading of fantasy. This is a misclassification on the same level as saying that Beckett's work is similar to Arthur Miller's because both of them wrote plays. Tolkein is concerned with elves, and Peake is concerned with the darker aspects of human nature, obsession, overweaning ambition, the desire for vengance, melancholy, madness, alienation, and so on. His characters have been described as Dickensian grotesques, but often they are far more developed than that, even though they are much more bizarre than the characters that Dickens created. Can Steerpike, Prunesquallor, Fuschia or Flay honestly be classified as a grotesque, when they exhibit so many different facets to their personality? Steerpike in particular is fascinating. Though he is an outright sociopath, he is not without motivation, or some level of justification, in his actions/reactions to his environment. A doctoral thesis could be written about what Steerpike tells us about the nature of human evil alone.
In addition to his remarkable gift for characterization, Peake is an accomplished poet, capable of incredible flights of fancy infused with a sense of doomed romanticism, which no other fantasist comes close to emulating. Everything in Gormenghast is decaying and ancient, and this decay is rendered palpable through Peake's verbal alchemy. The plot is wonderful as well, and Peake pulls off moments of surreal beauty (such as the depiction of the flooded castle and the hunt for Steerpike in boats), edge of your seat suspense, harrowing descents into madness worthy of King Lear, and absolute heartbreak which moved me to tears.
In short, the Gormenghast Trilogy, in spite of its obscurity and classification as "fantasy", is one of the major literary accomplishments of the 20th century. If you love literature, don't miss out on it.