Trøjborg Slotsruin is located approximately 12 km north of Tønder in Visby in southern Jutland. The name Trøjborg is probably derived from the Greek Troja.
The castle was built in the 1300s on a 30 × 30 m bank. In 1347, Trøjborg is mentioned in a letter from Duke Valdemar of Southern Jutland as Duke Valdemar Atterdag's castle. In 1407 it came into the possession of Queen Margaret, but later it was mortgaged to Ribe diocese.
After the Reformation in Denmark in 1536, the clergy were granted the crown, and King Frederick II transferred the castle to Daniel Rantzau as a thank you for his profits in the war against the Swedes. Around 1580, Peter Rantzau demolished the original medieval castle to erect a Renaissance castle on the site instead. When the Rantzau genus became extinct in 1658, the castle slowly decayed.
In the first half of the 19th century Johan Ferdinand de Neergaard was the owner. In 1851, Trøjborg was purchased by farmer Knud Lausten Knudsen, who had plans to set up a teacher's seminary on the site, but when these plans were not realized, the castle was demolished in 1854. Parts of the south wall and basement have been preserved.