Tier A · Early and mid game
Base Defenses
Reliable defenses protect your economy while your army handles the wave. Upgrade bottlenecks instead of improving every structure evenly.
What is verified
The official game description confirms base defense upgrades and expansion.
Practical rule
Upgrade the point where zombies consistently break through. If the line already holds comfortably, put the next investment into income or fielded damage.
Next step
Use the upgrade planner with your live income rate, then compare this priority with the other entries in the full list.