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?
''The building is abandoned, and it's beautiful, and it's eerie, and it was built to intimidate,'' said Sean Kelley, director of public programming. ''People travel from all over the country to come here for Halloween.''
As daring souls slink and cringe their way through the decaying property, deranged prisoners accost them for stepping on the wrong turf; overwhelmed guards scream for help; infirmary patients howl in pain under the care of disturbed doctors. In a psychedelic 3-D room, what looks like a wall ... is not.
For the easily frightened, there has always been some measure of comfort knowing that the actors are not allowed to actually touch them. Yet this year, the bravest visitors can opt for a glow-in-the-dark necklace that indicates their willingness to interact with performers.
City resident Raj Kumar, who wore the so-called zombie bait, said he got squirted with water while his wife was pulled through a secret tunnel.
''It's much more nerve-racking once you have the (necklace) on and you know people are sneaking up on you,'' Kumar said.
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.
Amy Hollaman, the show's creative director, said planning goes on year-round and sets are built months in advance. And each evening just before dark, about 130 performers converge on a makeup and costume room to be turned into gruesome characters.
Actress Jude Feingold, who has regularly performed Shakespeare, was happy to play an ax murderer on one recent night. Now in her fourth season at Eastern State, Feingold said she returns each year because of the great cast and crew - and for the satisfaction of scaring big, tough guys in baseball caps.
''I think it has a really good spirit,'' she said.
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.
Hollaman once heard a series of unnerving, unexplained noises while working late a few years ago. Petrified and unable to speak, she left immediately.
''Thousands of people ... have lived and worked here. There's been a lot of intense experiences inside this building,'' Hollaman said. ''It's hard to imagine that they haven't left a trace.''