I'm thinking the teaser is leading us to believe that a bunch of walkers are behind the first door the governor's people open at the prison. Isn't that a bit too easy, too expected?
If walkers are in the prison, Ricks group would have to let them in and contain them. If they did that, it would be closer to the breach on the other side, or at least on that path. So, if the governor's group is to be surprised by walkers, they'll have to be further within the prison.
OR
Tyreese has told them about the breach (remember they arrived that way) and they are prepared to fight through a bunch of walkers, hmmm, I haven't fully thought out that scenario. I don't think Tyreece would give out that info, o should explore that possibility.
Good stuff.
You should be a screen writer. LOL