The easter egg on level 7 was hilarious. It's amazing how great efforts in game design can come together with music (read: full orchestra) to create a game as relevant today as it was in 2007. The details are astonishing.
Note on level 13: If you cross the border early on, the second half should be a breeze. The upgraded Para 98 also helps, as others have pointed out.
Muahaha xD seems like u gotta make this into a challenging game youreself to make it any fun:P I just came to lvl 10 without buying anything at all :PP
 ":the game:"){kind=icon}
 "Extreme Missile Defense X-treme 3D"){kind=icon}
 "All That Matters"){kind=icon}
 "Headless Zombie"){kind=icon}
 "Magnetized"){kind=icon}
 "You Have To Burn The Rope"){kind=icon}
 "Vox Populi, Vox Dei(a werewolf thriller)"){kind=icon}
 "Icycle"){kind=icon}
 "UPGRADE COMPLETE!"){kind=icon}
 "run.don't.stop"){kind=icon}
 "Rotatix"){kind=icon}
 "Body Ladder"){kind=icon}
 "wOne"){kind=icon}
 "LOLcaptions"){kind=icon}
 "- Shape Invasion -"){kind=icon}
 "Medieval Rampage 2"){kind=icon}
 "The Mouse 101"){kind=icon}