you need 15 stars to unlock heroes..i don't reccoment you this but you can do this if you reset points on "Upgrade" section and go to hero room to use unlock heroes...15 stars will be lost with no chance to recall them so be careffull if yuo want this.You can keep(without premium content) 3 top heroes.The other six(and probably better) only with premium.Check it out and if you want pm me.
 "Kingdom Rush"){kind=icon}