Around 1076 Archbishop Gebhard of Salzburg, a follower of Pope Gregory VII in the Investiture Controversy, had the Petersberg fortress erected above the town in order to prevent Emperor Henry IV from crossing the Alps. The archbishop also had fierce enemies in the Carinthian ducal House of Sponheim, who after his deposition made several attempts to take possession of Friesach. Constant attacks by Duke Engelbert were finally repelled in 1124. In 1149 King Conrad III of Germany stayed at the castle on his way back from the Second Crusade, as did Richard the Lionheart returning from the Third Crusade in 1192, attempting to elude the guards of Duke Leopold V of Austria.
Today the castle is home to the Friesach City Museum, which features exhibits about the town's history, culture, mining industry and trade.
The Basilica of Santa Maria in Trastevere is one of the oldest churches of Rome. The basic floor plan and wall structure of the church date back to the 340s, and much of the structure to 1140-43. The first sanctuary was built in 221 and 227 by Pope Callixtus I and later completed by Pope Julius I.
The inscription on the episcopal throne states that this is the first church in Rome dedicated to Mary, mother of Jesus, although some claim that privilege belongs to the Basilica di Santa Maria Maggiore. A Christian house-church was founded here about 220 by Pope Saint Callixtus I (217-222) on the site of the Taberna meritoria, a refuge for retired soldiers. The area was made available for Christian use by Emperor Alexander Severus when he settled a dispute between the Christians and tavern-keepers.
The church underwent two restorations in the fifth and eighth centuries and in 1140-43 it was re-erected on its old foundations under Pope Innocent II.