The fact that you can sell turrets for the exact same price you bought them for means you can make a huge maze with 2 exits, block one of the exits and before the penguin reaches the open exit sell the closed exit turret and put a turret in front of there path.
to beat the last level go in a corner, he will go in the middle, then go in the oppisite corner. he will then go in a corner. then go in the final corner. you can work out the rest.
 "No Time To Explain"){kind=icon}
 "The Company of Myself"){kind=icon}
 "Prose and Motion"){kind=icon}
 "When Penguins Attack -TD"){kind=icon}
 "Duck, think outside the flock"){kind=icon}
 "WASD Tutorial"){kind=icon}
 "Shootorial #1"){kind=icon}
 "The Last Stand 2"){kind=icon}
 "Open Doors 2"){kind=icon}
 "Seven Deadly Sins"){kind=icon}