You start the game with only 150 gold and for each enemy you destroy you will get extra income. You only have 5 lives your base will not handle more then 5 enemies if you let more then 5 enemies to pass your defence you will lose and you have to restart your game.
National Defense is the right game for you to play on Gamerock.me.
If you want to play more games, check out Panda VS Zombies or Bunny Flags 2.
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Defense,Shooting,Action or just check out the best games page.