The conflict between Joanna and Pope Urban VI caused the Pope (as feudal overlord of the kingdom) to declare her dethroned in 1381 and give the kingdom to Charles. He marched on the Kingdom of Naples with a Croatian army, defeated her husband Otto, Duke of Brunswick-Grubenhagen, at San Germano, seized the city and besieged Joanna in the Castel dell'Ovo. After Otto's failed attempt to relieve her, Charles captured her and had her imprisoned at San Fele. Soon afterwards, when news reached Charles that her adopted heir, Louis I of Anjou, was setting out on an expedition to reconquer Naples, Charles had the Queen strangled in prison in 1382. Then he succeeded to the crown.