Bellesguard

Completed in 1909, Bellesguard was built for a private client on the site of a 15th-century summer palace of King Martin I of Aragon, and Gaudí incorporated remnants of the medieval structure into his design along with a Catalan flag-topped tower and characteristic parabolic arches. Because it's tucked into the residential Sant Gervasi hillside, well off the Passeig de Gràcia tourist route, it draws a fraction of the visitors of Casa Batlló or Park Güell despite comparable praise from those who make the trip, and reviewers frequently describe it as an overlooked gem. The audio-guided tour covers both the architecture and the site's royal history in multiple languages. It suits visitors who've already seen Gaudí's headline works and want a quieter, more in-depth look at his output.