Passau, city (1994 pop. 51,041), Bavaria, SE Germany, at the confluence of the Danube, Inn, and Ilz rivers, near the border with Austria. It is a river port, rail junction, and industrial center; manufactures include beer, textiles, optical equipment, and tobacco. A Roman frontier outpost known as Castra Batava, Passau was made (738'39) an episcopal see by St. Boniface. The bishops of Passau were temporal lords of a substantial territory until 1803, when the bishopric was secularized and awarded to Bavaria; the diocese was restored in 1817. The Treaty of Passau (1552) was negotiated there between Maurice of Saxony and King (later Emperor) Ferdinand I, who represented his brother, Emperor Charles V; it secured the release of the captive Protestant princes and helped pave the way for the religious peace of 1555. Noteworthy buildings in Passau include the cathedral (15th'17th cent.), which has one of the world's largest church organs; the Gothic city hall (begun 1398); the baroque episcopal palace; the Oberhaus fortress (13th'16th cent.); and a former Benedictine monastery (founded in the 8th cent.). There are also many fine houses and fountains in the Bavarian baroque style, built after fires ravaged the city in 1662 and 1680.