The Monastery of St. Michael on the Heiligenberg (Saints" Mountain), was a branch of the nearby Lorsch Abbey. The ruined complex that can be seen today was built beginning in 1023. Within the nave are traces of the Roman temple of Mercury. The monastery was abandoned in the 16th century.
The first mention of the monastery is in the Lorsch codex, from the 12th century, which dates the founding of the monastery to 870. No remains of an earlier monastery have been found, so this might have been merely an estimate of the founding.
References:Duino Castle was built by the Wallsee family in 1389 on the cliffs overlooking the Gulf of Trieste. It replaced an older castle from the 11th century. Over time, the Wallsee family disappeared and the castle, after having been used as a prison, became the residence of the Luogar and Hofer.
At the end of the 19th century it became the property of Prince Alexander von Thurn und Taxis from the Czech branch of the House of Thurn and Taxis. It remains with the family to this day with his great-grandson Prince Carlo Alessandro della Torre e Tasso, Duke of Castel Duino the current owner. The castle has been opened to the public as a museum and park.