Beating impossible
Don't evolve until you get to the stage where your club men are going against the enemies evolution stage of marines/knifeman/tank. You will have a lot of money by then cause cave man are so cheap.
Build many as fast as you can around 10-15 paces from your fortress as quickly as possible. The cave men (the club ones worth 15 bucks) will overlap giving a devastating blow. Think 8 club swings at once which can kill a knight...neh even a knight on his horse instantly. Make sure you stack up on a lot of club men before the archers and the rifle men (in the next evolve stage) get within projectile distance of your tower defense. I start with the egg thrower and slowly add turret spots. Add more turrets later and this just gets easier.
In the next evolve stage where the marines come in, unfortunately the cave man strategy doesn't work because the marine shoots once killing like 4 cave men at once. You will have to evolve now and if you did it right you should already have enough money to upgrade your turrets and mass buy men. Continue this strategy and use your specials as soon as you can. Remember that the enemy can only attack one man at a time so if your men overlap and some tank shoots you only 1 guy dies. Hint Hint. Those stupid marines with them machine guns are evil!
However you can probably beat the game before the marines even come. I never tried it cause I wanted more money! I basically had like 3/4 of the screen full of cave men and even with the enemy shooting those powerful cannons that blow up into many chunks which hurt your guys, you will slowly still advance. The game will unfortunately lag terribly. Hope this helps you guys.
Neat game. I agree with the other reviews. More evolution stages would be better. Also maybe a special that allows you to create cave men at a even faster speed. XD . Who knew the oldest age could be so superior. See yah.