While I don't have a specific list of all his writings, his works are mainly focused on German history, particularly the Holy Roman Empire and the Habsburg dynasty. He was known for his detailed research and thorough analysis.
Here are some of his key works:
* "Philipp II., König von Spanien" (1903), a biography of Philip II of Spain.
* "Die wirtschaftliche Entwicklung Italiens unter den Normannen und Staufern" (1901), focusing on the economic development of Italy under Norman and Hohenstaufen rule.
* "Die Entstehung des europäischen Staatensystems" (1922), exploring the formation of the European state system.
* "The Holy Roman Empire", a comprehensive work on the history of the Holy Roman Empire.
You can find more information about his writings by searching for his name in online databases like WorldCat or searching for his works in university libraries.
Remember that this is not an exhaustive list of all his works. You can find more information on his specific writings by researching further.