The Church of the Forty Martyrs was built under the rule of Bulgarian tsar Ivan Asen II (1218-1241). A revered monument of the Second Bulgarian Empire, it was transformed into a mosque in the 18C, and later returned to the Orthodox Church after the liberation of the country. In this church, Ferdinand Saxe-Cobourg-Gotha, proclaimed Bulgaria's independence in 1908. Archaeological investigations have revealed the tomb of tsar Kalojan and other notables and nobles of the Second Empire. There are engraved columns honouring khans Krum (803-814) and Omurtag (815-831) and Ivan Asen II.