As you can image the opposite faction has to own the fortress if you are trying to do this exploit!
There are several ways into the fortress while the other faction holds it.
Flying mount, logging out in the fortress, etc. whatever it takes, you just need to be in the fortress before the battle begins. Get up to one of the cannons and try to hide somehow until the battle begins. After the timer got to 0, you start clicking like mad where the cannon usually appears. If you are fast enough you will get into the cannon as they seem to be neutral shortly after spawn (at least for alliance it works, but I think it's possible for both factions) and own it for your faction! If there are no people around you can just frag the other cannons or leave them alone.
I wouldn't do this if there are lots of people around because it's likely they write some tickets.
Have fun
by Martin240