Children's Castle (Lastenlinna) is a former children's hospital in Helsinki, Finland. It was part of Helsinki University Central Hospital. The hospital was established in 1917 in the Kallio district of Helsinki. The building was designed by the Finnish architect Elsi Borg. Its current building in Taka-Töölö was completed in 1948. Pediatrician Arvo Ylppö was the hospital chief of Children's Castle from 1920 to 1963. The hospital was discontinued in 2018 when the New Children's Hospital was opened.