Danger Above 6-4 (aka Hal's Prison) is the 4th level of Danger Above. It features the Boomerang Bird, whom the player must free (By killing all the pigs) in order to advance the next level, along with taking out the Pigs.
Launch the Yellow Bird so it slides on the ground and destroys the wooden square. The structure should wobble, then fall. If done right, all the pigs will be eliminated, leaving you with 3 stars.
If you replay the level you will notice that Hal is no longer in the cage.