The album is a notable departure from previous Nightwish releases, lyrically and musically. The overall musical scheme is slower and darker than the more light-hearted likes of Oceanborn and Wishmaster, with more emphasis on ambience and melodic leanings. It contains orchestral backing, an ethereal production, softer and less operatic vocal work from Tarja Turunen and vocals by Marco in songs like "Ever Dream", "Slaying the Dreamer", "Dead to the World", "Feel for You" and "The Phantom of the Opera"; furthermore, the lyrics depict the strongest concept album undercurrents yet. This could also be considered one of Nightwish's darkest albums. The album received strong positive reviews. Century Child is also the first album on which guitarist Emppu Vuorinen tunes his guitar to D tuning in order to match Hietala's D-tuned bass.
Spinefarm Records released a 2-CD special edition version of the album in 2002. It includes the CD, liner art with the band members' signatures, and a Video CD containing the music video for their cover of Gary Moore's "Over the Hills and Far Away". In addition, the booklet contains an ID code that allows the owner to download the entire album and three extra tracks from the Internet for a limited time. The bonus tracks are "Nightwish", "The Forever Moments" and "Etiäinen", all from the band's first demo (1996).
The album was certified 2x platinum in Finland with more than 60,000 sold copies, the second most sold Finnish album in Finland in year 2002. Century Child has sold more than 90,000 copies in Finland alone. Worldwide, it sold more than 350,000 copies between 2002 and 2003.