Olsztyn Castle (Polish: Zamek w Olsztynie) – castle ruins located in the Kraków-Częstochowa Upland, lying on the Trail of the Eagles' Nests – formerly protecting the southern border of the Kingdom of Poland. The ruins of the 14th-century castle are one of the biggest attractions of the area.
The castle, located on a hill, among limestone rocks, is part of the Trail of the Eagles' Nests. It belonged to a system of fortifications, built by King Casimir III the Great, to protect western Lesser Poland from Czechs, to whom Silesia belonged at that time. For some time, as a fee, it belonged to Prince Władysław Opolczyk. Taken away from him in 1396, the castle was then handed by King Władysław II Jagiełło to a local nobleman, Jan Odrowąż of Szczekociny. The castle was invaded several times by Silesian princes in the 15th-century, and with the advancement of warfare, its fortifications became obsolete. In 1655, it was captured by the Swedes, and since then, it became a ruin. In 1722, it was partly demolished, with bricks used to build a parish church at Olsztyn. Currently, only fragments of defensive walls remain. The most impressive still standing part of the castle is a 35-metre round tower, built in the 13th-century, which served as a prison.