SAN DIEGO (AP) — The zombies are adapting on "The Walking Dead."

The fourth season trailer debuted at Comic-Con showed off a breed of stronger, savvier undead — or walkers, as they're called on the hit AMC series — encroaching on the survivors at their prison enclave.

"The Walking Dead" producers were joined by such cast members as Andrew Lincoln, Danai Gurira and Norman Reedus.

In the footage shown at the pop-culture convention, the survivor's barricaded headquarters has been further fortified. However, increasingly more hordes are bombarding the gates, and they aren't spreading out anymore.

The clip also teased a catastrophic event in the prison with multiple survivors dead. At the end of the trailer, Reedus' Daryl is shown driving a car and overhearing a voice on the radio saying "those who arrive survive."