The Gormenghast Trilogy

AFTER LORD OF THE RINGS. THE BEST FANTASY NOVEL OF THE 20TH CENTURY ...

From Wikipedia: "Gormenghast is a fantasy series by British author Mervyn Peake, about the inhabitants of Castle Gormenghast, a sprawling, decaying, Gothic structure. Originally conceived as a single on-going novel, the series was ended by Peake's death and comprises three novels: Titus Groan (1946), Gormenghast (1950) and Titus Alone (1959); and a novella, Boy in Darkness (1956). Peake was writing a fourth novel, Titus Awakes, at the time of his death in 1968. The book was completed by Peake's widow Maeve Gilmore in the 1970s, but was not published until 2011 after it was discovered by their family.

Although the first two books do not contain any overtly fantastical elements, Gormenghast is almost unanimously categorised as fantasy because of the atmosphere and pseudo-medieval setting. The series has received widespread acclaim from the speculative fiction community and mainstream literary critics.