The Alba Carolina Citadel is a star-shaped Romanian fortress located in Alba Iulia.
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 guldens.
The citadel was built on the site of 2 other fortifications: the legionary fortress of Legio XIII Gemina (known as Apulum), as well as the medieval Balgrad citadel. The shape of the citadel, an iconic element of Vauban architecture, influenced the design of Alba Iulia's city logo when the city adopted city branding in 2014.
The citadel is named after Charles VI, known as Carol VI in Romanian, who was the Holy Roman Emperor at the time of the citadel's construction.