I have been to Sofia with my son (21 year old) and we took the free Sofia tour.
We have walked through the churches, mosque, and synagogue in the streets of Sofia. We felt the history of the empires and the atmosphere of the statues, buildings, palace, and mountain nearby. We heard the stories about Boris the 3rd, the monarch, the communists and world war 2.It was real pleasure to discover the known and hidden parts of the city. Our guide, Victoria knew the way how to merge history, art, architecture, communists, humor and lion stories in a very intelligent and interesting way.