Six royal tombs are scattered across land to the south east of the citadel on the other side of the Perfume River and one on the same side. They are monuments to nine of the thirteen rulers of the Nguyen Dynasty, mostly built during the Emperor’s lifetime. Although designed individuall y, sometimes by the intended occupant himself, they share certain design conventions.

Source: http://www.haivenu-vietnam.com/des-hue-royal-tombs.htm