This is a combined historic/contemporary cityview of Sofia, the capital and largest city of Bulgaria. The bottom part is a watercolor by Josef Oberbauer called "Sofia in 1907."
This is my only card from Bulgaria. Since it doesn't have an ID I think it must be from a private swap, but I don't remember it!
The Bulgaria post website has no stamp information in English. The stamp is dated 2006. Bulgaria is an International Postal Union member, but the 2006 issuing program isn't available on their website. So - another mystery!