Also known as Mosque of Hamza Bay, it has the second largest single-domed among the Ottoman mosques when built. In 1856, the mosque was badly damaged by fire and the Baroque- style ornamental mural on its interior walls probably dates to after the fire in 1859 or 1860. In 2008-2013, its building was re-restored with a grant from the European Union and turned into a Museum of Religions.
The present building of St Demetrius' Cathedral in Sliven was built from 1831 to 1834 on a former wooden church site. In 1966 the church was declared a cultural monument of local importance. In the spring of 1971, the National Institute of Cultural Monuments undertook the restoration and conservation of the iconostasis.
This is the only functioning mosque in the city, the name Eski means old in Bulgarian, refering to the long history behind its construction.