You have to deal with more than just the walkers you can see. They know you’re there, and more are coming! You have to keep moving or you’ll be overrun.

The bottom of the screen shows how many walkers are currently incoming. The number on the left shows how many turns it takes until they arrive.

Making noise attracts more walkers, adding them to the incoming wave. Every gunshot, explosions running past them and some noisy objects will increase the threat, so take your actions carefully!