Rupert spent the beginning of his teenage years in England between the courts of The Hague and his uncle King Charles I, before being captured and imprisoned in Linz during the middle stages of the Thirty Years' War. Rupert had become a soldier early; at the age of 14 he attended the Dutch "pas d'armes" with the Protestant Frederick Henry, Prince of Orange. Later that year he fought alongside him and the Duke of Brunswick at the Anglo-German siege of Rheinberg, and by 1635 he was acting as a military lifeguard to Prince Frederick. Rupert went on to fight against imperial Spain in the successful campaign around Breda in 1637 during the Eighty Years' War in the Netherlands. By the end of this period, Rupert had acquired a reputation for fearlessness in battle, high spirits and considerable industry.