Considering the resent events that happened in the world, we feel we must ensure you that Plovdiv and Bulgaria (in general) is a really safe destination. Bulgaria has just one terrorist attack in the last 25 years and it was part of the war between Israel and Palestine.
There are no active threats from radical Islamic terrorists. Plovdiv is a multicultural city, we have a huge mosque in the center of the city (one of our landmarks, that you will see during a tour) and a very peaceful muslim community.
Bulgaria is part of NATO and EU and uses the intelligence agencies of all the allied countries to protect itself from terrorists. There is a much bigger threat in visiting Greece, France, Turkey or Germany.