Trịnh–Nguyễn War
In 1627, when these ramparts were completed, Đào Duy Từ presented a double-bottomed tray to the Lê Emperor in which he concealed the royal decree that demanded the Nguyễn lord to submit to the Lê emperors. He then carved the tray with a cryptic poem that involves a letter play, upon solving would reveal the message: 予不受勑, meaning Lord Nguyễn Phúc Nguyên does not accept the decree. He topped the tray with gifts and sent an envoy bringing it to Thăng Long. Unaware of the hidden content, Lord Trịnh Tráng accepted the tray and Tu's letter remained undiscovered until the Nguyễn envoy had fled back to the south. After learning the true meaning of Đào Duy Từ's poem; Lord Trịnh Tráng got angry and sent a large army to the south, starting the Trịnh–Nguyễn War.