From the World Heritage inscription:
The Transylvanian villages with fortified churches provide a vivid picture of the cultural landscape of southern Transylvania. They are characterized by the specific land-use system, settlement pattern, and organization of the family farmstead units preserved since the late Middle Ages, dominated by their fortified churches, which illustrate building periods from the 13th to 16th centuries.
In the 13th century the kings of Hungary encouraged the colonization of the sub-Carpathian region of Transylvania (Erdely) by a German-speaking population of artisans, farmers and merchants, mainly from the Rhineland. Known as the Transylvanian Saxons, they enjoyed special privileges granted by the Hungarian Crown, especially in the period preceding the creation of the Austro-Hungarian Empire. Despite living in a country where the majority of the population was ethnic Hungarians or Romanians, the Transylvanian Saxons were able to preserve their language and their customs intact throughout the centuries. Their ethnic solidarity is vividly illustrated by their settlements, which remained resistant to external influences
Their geographical location in the foothills of the Carpathians exposed the Transylvanian Saxon communities to danger when the Ottoman Empire began to menace the Austro-Hungarian Empire. Their reaction was to build defensive works within which they could take shelter from the invaders. Lacking the resources of the European nobility and rich merchants, who were able to fortify entire towns, the Transylvanian Saxons chose to create fortresses round their churches, enclosing storehouses within the enceintes to enable them to withstand long sieges. The first documentary reference to Biertan dates from 1283. In 1397 it was raised to the status of Oppidum (fortified town) and twenty years later the Hungarian King granted it droit de l’épée (jus gladii ), i.e. the right to bear arms. From 1572 to 1867 Biertan was the See of the Evangelical (Lutheran) Bishop of Transylvania, and as such played a major role in the cultural and religious life of the considerable German population of the region.
While there are technically only seven churches which are included in the world heritage list, there are many churches of very similar architecture and age located in Transylvania. Many of the churches in the region have had little to no attempts to preserve them. I visited several churches which were looted after the fall of the communists and have had no attempts at conservation. The fortified churches were designed to protect the villagers from the many invading armies which passed through the region.
View my complete list of UNESCO World Heritage Sites.