In my experience, I've taken down the door 4 to 5 times with Q9. With his huge range only a few castle defenders will be able to shoot him, his attack is decent if you can get height advantage on the door. I leave ALL of my units waiting to enter the castle out of range of the figures on the walls, only when the door is down do they move in range, and a lot of the time I'll use Q9 to pick off several guys on the walls with his special first. His defense is good, and with a life of 4 on top of heavy defense, he's hard to take down, so even if you have to take some fire while shooting the door, you usually end up ok.
Another technique that works pretty well, and I haven't heard many people mention this one: Use a flyer to get inside the walls and occupy the door space, then you can open the door from the inside and let everyone in. Many times if someone knows that they are going to bunker down in the castle and just shoot from the ramparts, they won't even bother having a door-guard. You can fly right in, they think that you are going to attack the people on the walls, but fly right by them and open the door.
For this, you have to have all your guys right outside the door, ready to move in as soon as it's open, because the moment that the castle defender sees what you are doing they will shoot your flyer down like there's no tomorrow. So using Raelin with her defensive aura to get everyone close to the door, then when everyone is in position you fly over and open it. Trust me, it will **** off whoever you are playing, but it's a great way to storm the castle.
