Task: Question Generation
Input (English): The architectural history of Estonia mainly reflects its contemporary development in northern Europe. Worth mentioning is especially the architectural ensemble that makes out the medieval old town of Tallinn, which is on the UNESCO World Heritage List. In addition, the country has several unique, more or less preserved hill forts dating from pre-Christian times, a large number of still intact medieval castles and churches, while the countryside is still shaped by the presence of a vast number of manor houses from earlier centuries.
Output (English): When were the preserved hill forts in Estonia built?