The son of Johan Castberg, he served as professor and rector at the University of Oslo as well as president of The Hague Academy of International Law.

He left his post as research fellow in 1924, having been engaged as a secretary and advisor in the dispute between Norway and Denmark over Greenland.

During his time as rector, political science was introduced as a separate discipline, being given its own department.

From 1962 to 1976 he presided over the curatorium of The Hague Academy of International Law,[1] an institution seated in the Peace Palace together with the International Court of Justice and the Permanent Court of Arbitration, and currently presided by Boutros Boutros-Ghali.

[6][7] Among Castberg's most important publications in this period are La philosophie de droit (1970), Den europeiske konvensjon om menneskerettighetene (1971), Minner om politikk og vitenskap fra årene 1900–1970 (1971) and Rett og revolusjon i Norge (1973).