Taking taxi will save you time. We took a taxi to Hissarluka fortress, the taxi driver waited for us and drove us to the town back. As a local guy he told us about some places of interest in the town. He also stopped several times for taking pictures.
The trains from Sofia to Kyustendil leave from the central train station in Sofia. For the current timetable please visit the official site of Bulgarian State Railways. It takes about 2 hours and a half to get there.
The Bulgarian Jews should have been deported from this trainstation.
After leaving the town of Zemen (about 2 hours from Sofia) look outside - the views of Zemen Gorge are magnificiant!