Answer
Hugo Grotius was imprisoned in Loevestein Castle in the United Provinces in 1619. He was imprisoned for his support of the Remonstrants, a group of Protestants who opposed the Calvinist orthodoxy of the Dutch Reformed Church. Grotius was held in the castle for 11 months, during which time he wrote his magnum opus, De Jure Belli ac Pacis, or The Law of War and Peace.