Making walls for the your base is CRUCIAL. You want to survive against the undead. Not die. First, make the walls similar to this.

Defenses will help A LOT as well. You're gonna wanna put the cannon setting as this: Now, you're gonna have to put them right infront of your base like this:

You can't have a zombie survival without the zombies to survive against! First, you'll wanna make them spawn forever. Select the Zone, then put the settings like this: Now, place them like this: Now the zombies will spawn!

Well, I hope this helped you!