Hauska Castle in the Czech Republic. It's rumored that this castle has an entrance to hell buried somewhere under the Church on the castle's grounds. It's also said that the castle itself is very haunted.

No building or site is haunted, it is the PERSON that is haunted. A haunting requires energy, and only a living being has energy to manifest a haunting or a possession. Is this castle haunted, most likely not.

Is there a gateway to hell somewhere on the castle grounds? That I can't speak to, but there are stories that the locals tell that misfortune comes to anyone that stays at the castle... they all die.

Historically, on a similar note, the Nazis had possession of this castle during WWII, and they were doing things that had something to do with the occult within the castle. They had found evidence of it in the castle after the war. I leave that up to you to make your own minds about.