Cassel became a student at the Berlin University, where he attended the lectures of the orientalist Julius Heinrich Petermann, the philosopher Friedrich Adolf Trendelenburg, the philologist Philipp August Boeckh, and others.

He, besides, maintained very friendly relations with Moritz Steinschneider, Heimann Jolowicz, Leser Landshuth, and Paul de Lagarde.

During the whole time of his university studies he supported himself by giving lessons; and having thus experienced all the bitterness of poverty, he became later one of the founders of the Hülfs-Verein für Jüdische Studierende, a society for assisting poor Jewish students in Berlin, which is still in existence.

Cassel began his career as an author with his doctor's thesis on "Die Psalmenüberschriften" (published in the "Literaturblatt des Orients," Leipzig, 1840).

Zur Verständigung über Jüdische Reformbestrebungen," Berlin, 1845; Cassel is also the author of all the articles dealing with Judaism and Jewish literature in Brockhaus' "Konversations-Lexikon".