jobbertje, to beat level 13, remember that the "defensive turrets" have a maximum range. At the beginning, choose an "arm" to start from and destroy only the outermost turrets on that arm (about half). Then circulate clockwise or counterclockwise to another arm and repeat. This will keep you out of range of most of the other turrets. If you need to, just back off to take care of the mobile attackers. Then you'll only have a small clump in the middle to deal with. In short, level 13 is all about patience.
 "Deep"){kind=icon}