>>The City and team eventually settled on building at the South Philadelphia Sports Complex, on the site of an old food warehouse much like Lincoln Financial Field.<< From Wikipedia.
But I also saw on Yahoo! this particular post by someone and maybe that's where you got the prison idea:
>>Citizens Bank Park replaced the old Veteran's Stadium in 2004. This stadium had a small jail cell in the basement to keep rowdy fans there during a ball game, the only stadium in league history with its own jail cell. The stadium also had its own judge during home games who had violent fans detained on the premises! Philadelphia fans are known to be some of the most violent fans, so this I believe.<<