I found this today while doing Twin Peaks with friends. I didn't see this anywhere here and this is an excellent way to win this Battleground in Rated Battlegrounds next week when it's 10-man week. This isn't hard at all to do, and is very doable alone. I tested wit h3 mounts as a undead and it worked for all of them. This is for hordes, but will work for alliances (since it's in the Horde base, this wouldn't be as useful for alliances.)
1. Face the entrance bridge of the Horde Base, get in the water and get as far as possible under the "floor".
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2. Mount up and Jump (the usual Mount Glitch).
3. At this point the flag dropped. So when you are on the water surface (which is hidden from other people), get your flag back; it will be where you mounted at.
4. You can get on the spot we can see on the screenshot to be able to cap the flag when it gets returned. It's a ledge on the ground that will allow you to pop out of the building whenever it's needed.
Have Fun and Good Luck in rated Battlegrounds!