Where: Büren, Germany
Construction: 1603-1609
Most important renovation: 20th Century
Accessible: yes, two museums
Important fact:
The only triangle shaped castle in Germany.
Used as an SS castle from 1934-1945. Partly blown up by the SS in 1945, but rebuilt after the war. It now houses two museums and
a youth hostel.
https://www.wewelsburg.de/en/