Hazmburk (German: Hasenburg, literally "Hare Castle") is a mountain peak in the České Středohoří range found in the Czech Republic, near the town of Libochovice. At the top of the mountain there is the ruin of a mediaeval castle, of which two towers and some wall fragments are still standing. A smaller first castle was already built there in the late 13th century by the Lichtenburg noble family. Later, the site was home to the Zajíc family: in 1335, Zbyněk Zajíc, a powerful aristocrat during the times of Charles IV., purchased it, and made it the center of his estates. In Czech, the word zajíc means hare, so the name of castle and hill is a mediaeval play on the name of its most prominent owners, who even had hares in their coat of arms. Zbyněk Zajíc considerably enlarged and extended the castle, and in particular added the two prominent towers which survive to this day.
The castle was never taken by force, despite multiple siege attempts during the Hussite wars. After these wars, it apparently fell into disuse and gradually was reduced to a ruin: records from the year 1586 already describe it as desolate. During the period of romanticism, the ruins became a source of inspiration for writers, most notably K. H. Mácha.
The castle site is open to the public in summer, and can be either accessed from the nearby village of Klapý, or from the train stop Slatina pod Hazmburkem. The taller of the two towers features an observation deck, and the view of the surrounding countryside that can be had from there is quite spectacular. The other tower is, while also in reasonable condition, not accessible to the public.