Rýzmburk, Castle ruin in Žernov, Czech Republic

Rýzmburk is a castle ruin standing on the left bank of the Aupa River in the Babiččino údolí region of Eastern Bohemia. The stone remains show traces of medieval fortress architecture, with foundations and fragments still visible among the hillside landscape.

Rýzmburk was first mentioned in writing in 1319; its first known owner, Albert of Rýzmburk, fell at the Battle of Crécy in 1346. The castle passed through several noble families until Vilém of Talmberk, its last resident lord, died and his brother Jan sold the estate on 12 April 1601 to Zikmund Smiřický of Smiřice. Unmaintained thereafter, it was described as deserted in 1628 and plundered by imperial troops in 1634 and by Swedes in 1641.

The German name Riesenburg, meaning "giant castle", survives in the Czech form Rýzmburk. The castle gave its name to the surrounding settlement and to the noble family of the Rýzmburks, whose member Albert fell at the Battle of Crécy in 1346. In 1798 Duke Peter Biron of Courland had a pavilion built inside the ruin.

The ruin is freely accessible for viewing, with no gate or ticket office; the remains lie on the left bank of the Aupa in the Babiččino údolí area, east of Žernov and south of Červená Hora. Only terrain relics of the fortifications and faint masonry fragments survive, so the ground is uneven and partly overgrown.

Stone from the ruined fortress was later reused in building the nearby Ratibořice Castle. This shows how medieval ruins often became quarries, with their material passing into new structures rather than being lost.