You have to fire it through that path after the pall has come down through the channel to the right hand side of the right flipper also, in normal play going up there does something else.
To open the door and initiate the siege you have to hit the secret entrance next to the left hand side tunnel, if that makes sense. It's the one you come out of every time a new ball is fired on to the table. o. Every time you do this that log of wood, whatever it's called, will move closer to the door. Do this 3 or 4 times and the door opens up.