In the middle of the city of Aiud is preserved one of the oldest urban fortresses in Transylvania-Romania (XIII-XVI centuries), the Aiud Fortress. The fortress is a historical monument, with the code AB-II-a-A-00172.
The current Citadel has as main components the Calvinist Reformed Church and the Evangelical Church, surrounded by a fortified enclosure. The fortress was built in the 14th century, due to its current appearance to the changes of the 16th and 17th centuries.