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.|
When you go through the door you'll initiate a challenge which involves a load of archers and a canon that you have to load by hitting the right hand side ramp, over and over. The canon should destroy the castle with the door, but i've never completed the mission.
The beast appearing instead of the dragon is a Knights Tale, I do believe. Never completed that one either.
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