From the World Heritage inscription:
Located on the outskirts of Sofia, Boyana Church consists of three buildings. The eastern church was built in the 10th century, then enlarged at the beginning of the 13th century by Sebastocrator Kaloyan, who ordered a second two-story building to be erected next to it. The frescoes in this second church, painted in 1259, make it one of the most important collections of medieval paintings. The ensemble is completed by a third church, built at the beginning of the 19th century. This site is one of the most complete and perfectly preserved monuments of east European medieval art.
In terms of total area, the Boyana Church is one of the smallest World Heritage sites I have ever visited. Maybe the smallest. The building itself is very unimpressive from the outside. The real treasure, however, is the frescos on the inside of the building. Getting to the church isn’t difficult, but it does require a bit of work. It is on the outskirts of Sofia. There are some mini buses which go there, but I wasn’t able to find them. I eventually just got a taxi which wasn’t that expensive. It is a worthwhile stop if you are visiting Sofia and due to its size, doesn’t require a lot of time to visit.
View my complete list of UNESCO World Heritage Sites.