Boštanj Castle, Castle ruins in Grosuplje, Slovenia.

Boštanj Castle, called Weissenstein ("white stone") in German, stands on a hill south of Grosuplje on the site of a prehistoric hillfort. It was built as a four-winged Renaissance structure with fortified walls and a defence tower in the outer bailey. Today only the northern defence tower survives, along with the north wall of the main building, which keeps an antique clock mount, a partly preserved portal, carved window frames, loopholes and stone steps.

The Lamberg family likely built on a prehistoric hillfort site in the second half of the 15th century. In 1549 Emperor Ferdinand I allowed Jakob von Lamberg to build a castle on the old Zagradec fortification; by 1558 it was enlarged and ringed with walls and defence towers. The Ursini-Blagaj family bought it in 1678 and held it until 1901, when Baroness Terezija Lauer-Lazarini inherited it; the Lazarini family kept it until 1942. Slovene partisans burned it in 1944.

The castle's German name Weissenstein means "white stone", while the Slovene name Boštanj links it to the nearby settlement. The Ursini-Blagaj family, owners from 1678, gave the estate to Baroness Terezija Lauer-Lazarini in 1901; the Lazzarini family held it until 1942. Today only the north defence tower and parts of the north wall remain, and people still live beside the ruin.

The hill of Boštanj Castle is the natural northern boundary of the Radensko polje, on Veliki Mlačev south of Grosuplje. Only the northern defence tower and the northern wall of the main building survive, so expect rubble and no marked trail or signage. People still live beside the ruin, so keep to the ruins and do not cross private ground.

The hilltop was a prehistoric hillfort before the Lambergs built here, and Emperor Ferdinand I authorised Jakob von Lamberg in 1549 to erect a castle on the old Zagradec fortification. Only the northern defence tower survives intact; the north wall keeps an antique clock mount and a partly preserved portal.