The Fortress of Arad is a fortification system built in the city of Arad in Kingdom of Hungary, on the left bank of the Mureș River in the 18th century at the direct order of the Habsburg Queen 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. It is the headquarters of the Romanian 191st Infantry Battalion which became part of the Mixed Romanian-Hungarian Battalion in 1999.