Well, you see. There were only two (2) zombies for this encounter. The first encounter featured a third, but others have also featured only two. However, in this case, the 5' width of the doorway, coupled with the party's close proximity (and ability to prevent movement from) the door, allowed only a single zombie to approach at a given time. While the same doorway interefered with our ability to damage the zombies in turn, it provided a much needed defensive boost that allowed us to severly weaken the first zombie before widening our formation to allow unfettered access . . .
oh, you didn't REALLY want me to tell you about it did you?