I've found only one reply about this level. I also have killed all the enemies & get all the keys I've found. BUT I still couldn't find the last key I need. I still couldn't fall to the bottom of the cave when the blue & green walls are open and get the last key I never got! when I press the down arrow, the rooster won't go down. How can I fall into the cave n' get the last key?
So, you can't fall down, eh? You have opened both walls. Only thing what you have to do is fall down on the place where green walls were. In the game you can run over 1 tile big hole, so try to walk slowly. That should work.