Seoninmun Gate (Hanja: 宣仁門) inside Changgyeong Palace is the place where King Yeongjo of Joseon had Crown Prince Sado-Jangheon a.k.a Posthumous King Jangjo locked up in the wooden rice chest inside the gate for 8 days; that resulting the death of the crown prince because of starvation and suffocation.
Crown Prince Sado-Jangheon (Born: 13 February 1735 - Died: 12 July 1762) was born as the second son of the Korean King Yeongjo of Joseon and father of King Jeongjo the Great, as well as being born the royal heir as a result of the early and youthful death of his older brother, Crown Prince Hyojang, in 1728; however, Sado was not given an opportunity to reign and was executed by starvation.
History indicates Sado suffered from mental illness; accused of randomly killing people in the palace and being a serial rapist. By court rules King Yeongjo could not kill his son by his own hands. As a result, Yeongjo, with the consent of Sado's mother, Lady Yi, issued a royal decree that ordered Sado climb into and be sealed within a large wooden rice chest on a hot July day in 1762. Crown Prince Sado died in 12 July 1762, eight days after King Yeongjo sealed him inside Seoninmun Gate.
King Yeongjo later became remorseful and gave his son the posthumous title 'Sado' ("thinking in sorrow"). It is often believed that Crown Prince Sado was a victim of a conspiracy by his political adversaries, but this is refuted in the Memoirs of Lady Hyegyeong, which was written by Prince Sado's wife Lady Hyegyeong.
During the 19th century, there were rumors that Prince Sado had not been mentally ill, but had been framed; however, these rumors are contradicted by his wife, Lady Hyegyeong, in The Memoirs of Lady Hyegyeong. Sado's death remains an issue of debate as to whether his death was a retribution for his actual misconduct or if he was the victim of a conspiracy by his political opponents.