An abandoned prison would seem creepy enough around Halloween. Now add blood-curdling screams and gruesome characters who can reach out and grab you.

That's the formula for "Terror Behind the Walls," the signature scarefest at historic Eastern State Penitentiary in Philadelphia, which is billed as the nation's largest haunted house outside an amusement park and staged for several weeks each fall at one of the city's most unusual tourist sites.

With its castle-like walls and decaying cellblocks, the deserted complex already conveys a particularly menacing air. What better place for gory scenes and sinister sound effects?

Eastern State Penitentiary was an architectural marvel when it opened in 1829, boasting indoor plumbing and heat even before the White House. Gangster Al Capone was among the most famous inmates before the prison closed in 1971. The site decayed for years before tours began in 1994.

"Terror Behind the Walls," which started 22 years ago, draws more than a thousand people on many nights. Proceeds provide about 60 percent of the annual budget for the property, which is now a National Historic Landmark.

Speaking of which: Are there really ghosts at Eastern State? Prison officials say people who study the paranormal believe the site is one of the most haunted places in the U.S.