Tap the mailbird flying around
Every now and then, you will see a bird dressed in a mailman’s uniform flying across the screen: make sure to tap it when it’s there because it will reward you with some free goodies. Not a lot, but every little bit counts!
On some rarer occasions, you will also get a flying drone. That is also a must tap item, which will reward you with much better loot, including power-ups, but the downside here is that you have to watch an ad to get it. Tottally worth it, though.
Watch ads to increase your income (but be selective)
Speaking of ads, you should know that after each successful mission, you are given the option to watch one and increase the amount of a specific reward: either the regular coins or the scraps and even the premium currency.
Keep an eye on this offer and don’t hesitate to take it if you need one type of resource and you need it fast. Also remember that the rewards are better the more advanced a stage is, so to max out your gains, grind on the latest stage you can easily beat.
Replay stages if things get tough (or to master them)
Zombie Idle Defense is a game that requires a certain amount of grinding: you will reach a point where zombies will be very difficult to beat. This usually happens soon after clearing New York City.
This is when you should get back and replay stages that you have already completed. Play them over and over again in order to grab some nice rewards and have enough resources to upgrade your characters or weapons.
If the overall power of your team has increased drastically, you can also try to approach levels that you haven’t managed to master (get 3 stars) in the past. This is really helpful as it will eventually increase the amount of resources you get in the idle mode.