Stephen I, the first crowned king of Hungary whose reign began in 1000 or 1001, unified the Carpathian Basin. Around 1003, he launched a campaign against "his maternal uncle, King Gyula" and occupied Transylvania. Medieval Transylvania was an integral part of the Kingdom of Hungary; however, it was an administratively distinct unit. In the territory of modern Romania, three Roman Catholic dioceses were established with their seats in Alba Iulia, Biharea, and Cenad.^[36]

Royal administration in the entire kingdom was based on counties organized around royal fortresses.^[37] In modern Romania's territory, references to an ispán or count of Alba^[38] in 1097, and to a count of Bihor in 1111 evidence the appearance of the county system.^[39] The counties in Banat and Crişana remained under direct royal authority, but a great officer of the realm, the voivode, supervised the ispáns of the Transylvanian counties from the end of the 12th century.^[40]

The early presence of Székelys at Tileagd in Crişana, and at Gârbova, Saschiz, and Sebeş in Transylvania is attested by royal charters.^[41] Székely groups from Gârbova, Saschiz, and Sebeş were moved around 1150 into the easternmost regions of Transylvania, when the monarchs granted these territories to new settlers arriving from Western Europe.^[42] The Székelys were organized into "seats" instead of counties, and a royal officer, the "Count of the Székelys" became the head of their community from the 1220s. The Székelys provided military services to the monarchs and remained exempt of royal taxes.