Crystal Mine

The Crystal Mine is a building that helps you give crystals, the main currency of the game. It is important to keep these up, as they fuel your base. You receive 2 at the beginning of the game and cost 0 crystals to build.

Tips

 * Always place as many crystal miners as you have, and max them out as fast as possible in order to level up your base quickly.
 * Keep strong defenses near your crystal miners, as crystals are necessary to upgrade your Base.
 * They are your most important building behind your Base.
 * These are your top priority when upgrading. Always keep these the same level as your Base for high income flow.
 * Upgrade these 2-3 levels higher than the rest of your base to increase Crystal income.
 * Don't place crystal mines near one layer walls, as suicide bomber zombies can easily destroy them.

Characteristics at different levels
 Original reference