Kyustendil (kyŏstĕndēlˈ) [key], city (1993 pop. 54,452), SW Bulgaria, near the Serbian border. Famous for its mineral springs used to heat hothouses, Kyustendil is a market city for fruit and other agricultural produce. There are varied light industries. The city's history dates to Roman times. It was the capital of an independent Bulgarian principality when the Turks took it in the 14th cent. The city remained under Turkish rule until 1878, when it became part of Bulgaria.