Kalec Castle, Castle ruin in Bač, Slovenia.

Kalec Castle is a castle ruin located near the source of the Pivka River at approximately 618 meters elevation. A single tower and sections of defensive walls remain standing against the hillside landscape.

Built in the mid-17th century by the Steinberg noble family, the castle was illustrated in Valvasor's 1689 Glory of the Duchy of Carniola. Wolf Ernest von Steinberg lost it in 1686 to Maria Anna of Auersperg; the Auersperg family held it thereafter. In 1786 it passed to Matej Vilhar, ancestor of composer Miroslav Vilhar, who died there. Abandoned after World War I, it fell into ruin.

The name Kalec (locally pronounced "kalc" or "kauc") was recorded in German as Steinberg or Stemberg. On 9 May 1869 Miroslav Vilhar, whose family owned the estate, organised a mass tabor rally here for a United Slovenia. The composer died at the castle, and his widow Marija sold the property in 1903 to nine landowners from Bač.

The ruins stand on the slope of Breg at 618 m, near the source of the Pivka River above Knežak. The Krpan Hiking Trail runs past the tower, and a chestnut-lined path leads up to the ruins, which sit among old linden trees. The climb is on foot over uneven ground; sturdy shoes help.

The TIGR meeting took place in 1941, not during the Second World War's later years: the antifascist group gathered here that year, before the wider conflict reached the area. The same tower hosted a mass rally for a United Slovenia on 9 May 1869, called by Miroslav Vilhar, whose family owned the estate and who died at the castle.