Kozlov, Medieval castle ruin in Košov, Czech Republic.

Kozlov is a ruined castle on the melaphyre summit of the same name in the Ještědsko-kozákovský hřbet, southeast of Košov near Lomnice nad Popelkou. It belongs to the shield-wall type: a single-part core enclosed by a rampart and ditch, with a curtain wall about two metres thick and lighter buildings attached inside, now visible only as terrain traces.

Kozlov was founded in the first half of the 14th century, as shown by pottery found on the site. The only reliable mention dates from 1462, when a description of the Lomnice estate already calls it deserted. A 1667 register records only a piece of the old wall.

The two chambers carved into rock show how residents adapted to this location by using natural stone formations. These spaces reveal how medieval builders worked with the landscape rather than against it.

A green-marked hiking trail leads to the ruins from Košov toward Peklov, and the nearest railway stop is in Cidlina on the Mladá Boleslav–Stará Paka line. The site has no facilities, so bring water and wear sturdy shoes for the forest paths.

A tunnel beneath the castle core, entered from the northeast, is 130–170 cm high and about 80 cm wide. It slopes down for roughly ten metres and ends in a steep, artificially cut shaft over eleven metres deep with a collapsed bottom. Tomáš Durdík linked it to water supply, while Zdeněk Fišera saw it as an escape route.