Complex sentence: Dag Hammarskjöld was born to the noble Hammarskjöld family in Jönköping, but spent most of his childhood in Uppsala, his home there, that he considered to be his childhood home, was Uppsala Castle.
Split into simpler sentences: Dag Hammarskjöld was born to the noble Hammarskjöld family in Jönköping. His home there, which he considered his childhood home, was Uppsala Castle.