I think we can definataly say that the zombies are prisoners because if you look at the map in the extended trailer/behind the scenes thing there are jail cells, and also a wardens office. It is definately well after the Jail opened, and also what if part of the easter egg is finishing the bridge. If in one scene it is under construction and another it is comeplete, then maybe the easter egg is comepleting it. Also someone on youtube mentioned that what if escaping alcatraz is the Easter egg. Maybe you finish the bridge and transport it somehow using magic or whatever then you escape
