The Alba Carolina Citadel is a star-shaped fortress located in Alba Iulia, Romania. Its construction commenced on November 4, 1715, during the Habsburg rule in Transylvania, and was completed in 1738. 20,000 serfs were involved in its construction, which is estimated to have cost around 3 million florins. The citadel was built on the site of two other fortifications: the legionary fortress of Legio XIII Gemina, as well as the medieval Belgrad citadel.