In 626 Li Shimin ambushed the crown prince
李建成 and his brother
李元吉 at the
玄武门 Xuanwu Gate, the north gate of the palace in
长安. Within about two months his father
李渊 had stepped down. The gate is named after
玄武 the Black Tortoise, guardian of the north. Historians still debate how far the official record, compiled under Taizong, shaped the story in his favour.
