The construction of the castle began in the same period as the construction of the church of St. George. The castle stood in the south-eastern corner of the city, as a three-winged object, closed on the western side by a curtain wall with a gate. The wing of the castle from the side of the courtyard was surrounded by a wooden gallery, in the courtyard there was a well. The most important part of the castle was its northern wing. The other wings served as economic functions. At the end of the fourteenth century, the castle was surrounded by walls with a gateway from the city side. The stronghold did not have a tower and ward that was characteristic of the Teutonic building. The dam of the Guber River was created by the Młyński Pond (buried in 1910) protecting the castle from the south. To the east, the castle was protected by a moat, which was flowing from the lake located north of the city. The castle in the 17th century was rebuilt many times. In 1945, it burned down. In the years 1962-1967 the walls of the castle were reconstructed on the basis of preserved engravings from the 19th century. The interiors are contemporary and adapted to the needs of the Museum and Library. The organizer of the Museum in Kętrzyn was Zofia Licharewa.