Thus, 20-year-old Frederick likely only moved into his residence at Eschwege after his marriage in 1646 in Stockholm with Eleonora Catherine, the sister of King Charles X Gustav of Sweden.

Due to his military career, he spent much time at the Swedish court and rarely visited Eschwege.

Frederick died on 24 September 1655 in Costian near Poznań, Poland, during the Second Northern War, in the army of his brother-in-law Charles X Gustav of Sweden.

His full title, as immortalized on his coffin, was: Frederick, the brave hero, Landgrave of Hesse, Prince of Hersfeld, Count of Katzenelnbogen, Diez, Ziegenhain, Nidda and Schaumburg.

Frederick married on 8 September 1646 in Stockholm with Eleonora Catherine (1626-1692), daughter of the Count Palatine John Casimir of Kleeburg (1589-1652) and sister of the Swedish king Charles X Gustav.