Apulum was a legionary fortress in the Roman province of Dacia from the 2nd to 4th centuries AD, located in today's Alba Iulia, Romania.
It is the largest castrum in Romania, occupying 37.5 hectares (93 acres) (750 x 500 m). It was the base of the legion Legio XIII Gemina transferred there by Trajan to the newly conquered province of Dacia at the end of the war in 106. In the era of Hadrian (117-138 AD) and of Antoninus Pius (138-161 AD) it was rebuilt in stone.
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Ulpia Traiana Augusta Dacica Sarmizegetusa was the capital, the first, and largest city of Roman Dacia, named after Sarmizegetusa the former Dacian capital, located some 30 km away. It was founded in 106 as a colonia deducta and elevated to metropolis during the reign of Severus Alexander.The present village of Sarmizegetusa has been built over parts of it.
It acted as the seat of the governor of Roman Dacia until it was moved to Apulum around 158 under Antoninus Pius.
Alba Iulia (Romanian pronunciation: [ˌalba ˈjuli.a] ; German: Karlsburg or Carlsburg, formerly Weißenburg [ˈvaɪsn̩bʊʁk] ; Hungarian: Gyulafehérvár [ˈɟulɒfɛɦeːrvaːr]; Latin: Apulum) is a city that serves as the seat of Alba County in the west-central part of Romania. Located on the river Mureș in the historical region of Transylvania, it has a population of 64,227 (as of 2021).
During ancient times, the site was the location of the Roman camp Apulum. Since the High Middle Ages, the city has been the seat of Transylvania's Roman Catholic diocese. Between 1526 and 1570 it was the capital of the Eastern Hungarian Kingdom from which the Principality of Transylvania emerged by the Treaty of Speyer in 1570 and it was the capital of the Principality of Transylvania until 1711. At one point it also was a center of the Eastern Orthodox Metropolitan of Transylvania with suffragan to Vad diocese. On 1 December 1918, the Union of Transylvania with Romania was declared in Alba Iulia, and Romania's King Ferdinand I and, in 1922 Queen Marie were crowned in the Alba Iulia Orthodox Cathedral.