The Fortress of Arad is a fortification system built in the city of Arad, Romania, on the left bank of the Mureș River in the 18th century at the direct order of the Habsburg Empress Maria Theresa. The fortress today lies in the city's Subcetate neighbourhood, on the former military border between the Habsburg Empire and the Ottoman Empire. Over the course of its existence, it has been used as a military garrison and a military prison. Since 1999, it has been the headquarters of the Mixed Romanian-Hungarian Battalion.