The biggest caveat with any casual free-to-play game is the premium currency system. Just about everything is on some kind of timer, and as you progress, those timers get longer. Don’t cave and spend your precious gems on speeding up production. Save them for something important that you get to keep. Make a point to complete achievements and clear natural obstacles to build your gem count. Above all else, gems should be spent on builders huts. After that, there’s a wide range of discretion on gem spending. Typically, buying resources outright with gems is a poor use of resources; you’re better off boosting your own production buildings.
When you do have enough gems, think long and hard about what’s important to you in the game, since if you want to get through this without paying dime, it’ll take awhile before you make another major purchase. On the flip side, you’ll have a hard time getting through the competitive end-game content without spending some cash.
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