Balmburg, medieval castle site, Cave castle ruin in Bachs, canton of Zurich, Switzerland

Balmburg, also called Grottenburg Hohfluh, is a ruined cave castle at 514 m on the west slope of the Sanzenberg above the Bachsertal. Its roughly 12 m deep cave lies at the foot of an overhanging Nagelfluh wall. The entrance was closed by a 1.8 m thick wall of boulders; a few metres survive at the sides, while only the foundation remains at the front. Tuff blocks above the soot-blackened cave mouth point to a former vault.

No historical records document the castle. It may have served a small clearing community as a centre, or formed part of a valley barrier in the Bachsertal mentioned around 1500; a link with Waldhausen Castle appears in folk legends. Finds from excavations in 1850 and 1878 indicate use into the Late Middle Ages. Blackened tuff at the cave's upper edge and a 25 cm coal layer point to destruction by fire.

The name Erdmännliloch, used on Swiss national maps, links the cave to legends of little earth men; folk tales also connect the site with nearby Waldhausen Castle. Excavations in 1850 and 1878 found small objects showing use in the late Middle Ages, and a 25 cm coal layer points to a fire.

The ruin lies at 514 m on the western slope of the Sanzenberg, above the Bachsertal near the Zürich–Aargau border. On foot it is about 15 minutes from the Thalmühle, with roughly 100 m of ascent. The cave is some 12 m deep and the ground in front of the surviving wall foundations is uneven; sturdy footwear helps.

The cave is about 12 metres deep, and its mouth was closed by a 1.8-metre-thick wall of boulders following the overhang's drip line. Only a few metres of wall survive at the sides; at the front just the foundation remains. Above the entrance, tuff blocks in the soot-blackened rock point to a former vault, and beam sockets are still visible in the rock.