All of those very ominous shots of walkers headed into Alexandria begins to really pay-off just past the midway point of the trailer, as they finally make their big move into the town. It's through the eyes of Michonne we first see this, hiding out and watching the ensuing danger. It's very creepily shot, and the low growling in particular is a great effect, before we then get a look at the walkers. After that it switches to Jessie and her son, who again are hiding out from the threat, which suggests that near enough the whole town has gone into some sort of shutdown mode while they work on a plan to either escape or defeat them.