Grinding for Silver to Get Past a Level

At some point in the game, you'll find yourself up against an enemy which you just can't seem to stop, so you'll really need to look at further upgrades to your towers and magic cards.

However this usually needs silver which you can get by completing levels, so you will need to go back and play over a few times earlier levels to gain enough silver to forge your cards by evolution or fusion.

The store also offers upgrade options which you can use stars or silver to purchase (as well as real money).

A little bit into the game a new level will fork off into three primezone raid levels and one proper level.

It's always good to fight through the three primezone raid levels (easy, medium, hard) to get enough cards and resources to help you continue through the game and build enough of a solid foundation for the later levels.

Once you have completed the actual level a new fork opens up and while you can go back and play that level again, you cannot play the primezone raids associated with that level again.