3rd Bonus Level:

A bit into level 4 you will see a group of enemies,
4 green and 4 red. The green enemies will appear 
first, and the red will follow behind. Wait until 
the red ones pass over the "pyramid"-looking 
blocks, just before they leave off of the top of 
the screen. Destroy the red ships at this point 
(the special multi-attack seems to work well). 
This will open the path to the third secret level
(a cool retro level--you should recognize it if you
played the original).