Haselburg, Castle ruin in Vimperk II, Czech Republic.

Haselburg, also called Hrádek or Kulatá Bašta, is a ruin on a rocky spur above the Křesanovský potok valley, north of Vimperk's centre. Its surviving two-storey round tower, 14 m across and roofed with cross vaults, was the forward bastion of Vimperk Castle, enclosed by a horseshoe-shaped wall.

Petr Kaplíř had the bastion built in 1479 as an artillery outwork for Vimperk Castle, whose position on the southern slope was unfavourable. In 1531 it briefly became the separate seat of Petr Malovec of Chýnov, who lost his estates in 1547 after the Battle of Mühlberg for backing the revolt against Ferdinand I. Demolition began after 1729 and was halted in 1732.

The name Haselburg alternates with Hrádek and Kulatá Bašta in local usage. Above the entrance sits a heraldic panel of the Kaplíř of Sulevice dated 1479, and the round tower's cross vaults survive. From the mid-18th to the 19th century, cottages were built around the tower.

The ruin is private property and not open to the public; it can only be viewed from outside. A red-marked hiking trail passes close by, and the street Zámek circles it on the other side. Two recreational buildings put up next to the bastion in 2014–2016 now partly block the view of the tower.

The round tower's diameter is 14 m, and above the entrance is a coat-of-arms panel of the Kaplíř family of Sulevice dated 1479. The tower's second floor originally served as living quarters. The site is privately owned and not open to the public.