Which Tolkien novel features the fictional kingdom of Gondolin?

The Silmarillion features the fictional kingdom of Gondolin.

Gondolin is a hidden Elven city of the First Age, founded by Turgon in a secret valley of Beleriand. Its story appears in the Quenta Silmarillion, the central section of The Silmarillion. The city remains concealed for many years before being betrayed and attacked by Morgoth’s forces. Its fall is one of the major tragedies of Tolkien’s First Age mythology.

Gondolin is also the subject of The Fall of Gondolin, a separate volume edited and published by Christopher Tolkien in 2018. That book presents different versions of the story from J.R.R. Tolkien’s manuscripts, but it was not one of the novels published during Tolkien’s lifetime. Readers sometimes associate Gondolin with The Lord of the Rings because both works contain hidden Elven strongholds, yet Gondolin belongs to the much earlier First Age. The Silmarillion is the principal published source for its place in Tolkien’s overall mythology.