Brežice's stately Renaissance fortress is both the main tourist attraction in the old town centre and the symbolic heart of the region, it is also fittingly home to the Posavje Museum as well as one of the Slovenia's most beautiful Baroque halls. Originally built by the Salzburg bishopric after it acquired feudal properties in the area during 11th century, the castle was first mentioned in writing in 1249 and by the middle of the 15th century it had come into the possession of the Hapsburgs. The following 100 years was the most turbulent in the castle's history, as its position at the frontier between the Austrian, Hungarian and Ottoman empires made it of great strategic importance.

After changing hands several times, it was badly damaged during the pan-Slovenian peasant uprising of 1515 before being completely destroyed during a Turkish incursion just over a decade later. It was at this time that the Hapsburgs, who had again gained control of the area, decided to construct a new, more strongly fortified castle, and engaged a team of Italian masters to build it. Their efforts were successful, as the castle was the only one in the Posavje region not to be overrun during the Croatian-Slovene peasant riots of 1573, and the its outward Renaissance appearance has been little changed to this day. During the 17th century the castle changed owners several more times, with the noble Attems family emerging as the eventual owners, which remained the case until the end of the Second World War. Since 1949 it has been the location of the main collection of the Posavje Museum, and also one of the main venues of the SEVIQC Festival of Early Classical Music.