I got this game years ago in early access. It now includes a campaign but it's essentially various maps linked together by a narrative.
I didn't think much of the campaign game but I still enjoyed building walled bases and the pressure of surviving the final wave - "Oh my god! They are billions!"
This mode is new. You have a limited amount of resources to defend an outpost as hordes of zombies come in all directions.
But this is much more fun... walled bases, choke points, masses of bodies.
The latest update added small machine guns which was useful too. They make a racket so it attracts more zombies into the vicinity, but the more tools made available, the more options you have to survive.
For now I think I've played enough of this game. It is also fun to play with a trainer.