Fredrick liked to hunt with falcons and that was his hobby during his life. Fredrick II wrote a book on falconry and he was very devoted to understanding falcons. His book about falcons was considered the first European example of falcon science. This falcon book is still important today.
Fredrick II designed a "Falcon's Tower" in his Castel Del Monte. The third tower which is located on the North side of his castle (West tower on that side) had upper levels that were falcon eyri (aerie) which are nests for his hunting falcons.
References:
http://faculty.ed.umuc.edu/jmatthew/naples/Fredrick.html