Hue is well known for its historic monuments, which have earned it a place in UNESCO's World Heritage Sites. The seat of the Nguyen emperors was the Imperial City, which occupies a large, walled area on the north side of the Perfume River. Inside the citadel was a forbidden city where only the emperors, concubines, and those close enough to them were granted access; the punishment for trespassing was death. Today, little of the forbidden city remains, though reconstruction efforts are in progress to maintain it as a historic tourist attraction.