The Imperial Citadel of Thang Long was built during the Ly Dynasty in 1009 and was used continuously by the Vietnamese royal families through the Tran, Le and up to the Nguyen Dynasty until Emperor Bao Dai abdicated and transferred power to the Democratic Republic of Vietnam in 1945. By the end of the 20th century, several of the remaining structures were torn down and only in the next century were the foundations systematically excavated. The centre of the imperial citadel was listed as a World Heritage Site by UNESCO in 2010.