Tip: At the start of the game, if the enemy ships are coming down the sides, just start over. Half your shots will miss and the ships will just sit there killing your population. Your cannons will also take massive damage which will require you to sell them and spend valuable money replacing them.
Why does every class of enemy, regardless of difficulty to kill, only give 4 resource points? If the price of more effective weaponry is increased, then the resource points for tougher enemies should be increased as well.