Little is known of the rock castle of Kozlov (otherwise known as Chlum). It is built on a quartz sandstone block, which has eroded into classic Český ráj pillars, gullies and towers, which were used to create the castle fortifications.

Kozlov was almost certainly founded in the early 14th century by a direct descendant of Jaroslav of Turnov, one of the key members of the Markvart family that dominated ownership of the region in the 13th and 14th centuries. The estate was absorbed into the larger territory controlled by the Valdštejn castle sometime in the 1 4th century. The castle was probably abandoned by the mid 15th century, having seen action in the Hussite wars.

It is a mysterious place, with hidden rock rooms and staircases. The paths to and from Kozlov are often deserted and it is a nice location to get a taste of the romantic Český ráj landscape without the tourist crowds.