An estimated 15,000 people will be partying in Germany's Burg Frankenstein through November 9th.
It's believed the 1,000-year-old castle may have inspired Mary Shelley's famous monster.
The Halloween parties at the castle started with American GI's who were posted in the area following World War II.
Scary costumes aren't required, but are encouraged.
Tourists now get to party at castle Frankenstein for about $25 each.