Trịnh–Nguyễn War
From this point on, Đào Duy Từ served as the chief military advisor, directing the construction of Nguyen's two famous strategic lines of defense: the ramparts of Lũy Thầy and Lũy Trường Dục, in northern Thuận Hóa (present day Quảng Bình Province). During the Trịnh–Nguyễn War, Lũy Thầy and Lũy Trường Dục were largely impregnable, enabling the Nguyễn lords to defend themselves against the Trịnh invasions, despite the population and army of the Nguyễn being smaller than those of the Trịnh lords.