The closest is Hofburg, Innsbruck, 1 km north-east of the city centre; the most famous is Neuschwanstein Castle (#8 of 9,035 worldwide) 58 km north-west. The oldest dated site in the circle is Sigmundskron Castle, founded 945.
The 100 km circle reaches into 4 countries: Italy (33), Germany (22), Austria (21), Switzerland (2).