In ancient times, this city was known as Olbia and Theodorias. Its modern name of Qasr means “castle of Libya”. The city was attacked several times by various invaders but was restored to its glory by the Byzantine emperor Justinian in 539 AD, who named it Theodorias, after his wife Theodora.
One of the most famous sites in the town is a 6th century Byzantine church, with mosaic floor panels, which are considered to be one of the best examples in the world of this type of art. The theme is mainly a mythical representation of the various spiritual beings of the roman era, as well as those of Christian symbolism, strongly indicating a time when both Paganism and Christianity were equally tolerated.