This is credited to 'Aneurism', from GameFAQs:
"Well, besides the monastery, there's the castle glitch. It's one I accidentally found when trying to get on to the single balcony on (from when facing from the entrance) the right.
To get to that area, you have to first fly to the portal behind the castle with Arokh. Once there, as Rynn, face the castle. Now, run along the bridge until you get to a chunk of stone connected to the bridge (there will be one on your left, and your right). Take the one that's on your right when facing the door into the castle.
Now, though the stone platform your standing on seems to only connect to a very thin and slanted line of stone sticking out from the castle wall. You can walk on it. Just creep around to the end of the stone line. The line should end at a corner. However, if you look down, you can see another similar line of stone below you.
Jump down to that line. Before continuing, take a look at how the corner ahead of you. You can't just run down the line and reach the corner. To get to the corner, you have to run down the line, and a bit before the corner you have to jump forward and land on or close to the corner. This is the only way to reach the corner. If you try to simply run to it, an invisible wall will pop up in front of you, also, if you run back after hitting the wall, another one will pop up behind you, thus boxing you in and forcing you to start over.
Alright! Your almost done getting into the castle. From the corner, head along the line to the next corner. Now, at this corner, turn Rynn to the right so as to face the castle. All you have to do now is roll forward while leaning against the castle wall. You should find yourself going through the wall pretty fast (I guess the developers forgot to solidify this part of the castle). Now you'll be inside the castle except it will look a lot like the monastery did when you glitched into it. From here you can run down into Zola Dane's secret dungeon, or you can even access the treasure room (I did this after the Shadowmire, so I was able to get the 9000 gold early on). Sorry, but it's been a while since I last did this glitch so my memory is fuzzy, but I think the way to exit the castle after grabbing the gold is to simply run through the door leading to the outside. From there, it's simply jumping from the balcony to a flat piece of land connected to the bridge that leads to the front door of the castle.
Oh, and I've heard of the Sunny Islands glitch Headhuntress discovered. I haven't tried it yet though, I'll probably do it on my next play-through."
Thank you, Aneurism!!
You can approach this glitch from the left hand side as well, btw.
Hopefully that will help some of you to achieve this glitch and maybe score on the cash early. Remember to save before attempting this, as there are a couple of areas inside which can either trap Rynn or drop her thru the floor, which she may or may not survive.