Very informative self guided audio tour available in English, Bulgarian and Russian from the audio guide desk at the Tourist Information Centre in the centre of the city by the Roman Stadium from where the tour starts. The advantage of an audio tour is that you can take it entirely at your pace, and even if like us you  didn't have time to visit all locations, we sat in a cafe and listened to the descriptions of the places we didn't get to. Free admission is included to a couple of sights as well as a discount voucher for the Puldin Restaurant in the Old Town.