Even native English speakers don’t know what that means.
Leave the castle maybe?
It is an odd phrase, but I always took it as
Now the siege is broken you can leave.
Basically you Break a Siege to be able to leave, so as a sentence it’s a little late. Firing the last cannonball to destroy the catapult actually Breaks the siege.
I guess it’s just a mistranslation. It should be Leave the Fort.
Mistranslation doesnt seem likely at all, the phrase carries over very literally with no real room for other interpretation…so it just sounds weird in either language.
More likely is that this is some alpha leftovers that never got cleaned and now only serve to confuse people 
In keeping with siege terminology it really ought to be “Sally forth!”
But I suspect even fewer “native English speakers” would understand that. And therefore it has my full support.
Breaking a siege is when you “sally forth” to attack those besieging to drive them off.
Perhaps some battlefield fight sequence was cut where there were more than a few enemies between you and the portal. I’m grasping at straws like the rest of you, but I suspect there was supposed to be an end-of-mission arena where you drive the attackers back from outside of the fort.
I’m just recalling the first time I did the mission a few years back and I thought, “Oh, it’s over?”.
Your interpretation does make some sense, i just get bugged by the fact that…isnt the siege kinda broken already if the attackers lose all their siege engines and their originally successful wall-breachers get KO´d?
All we do there is saunter over the bridge after the fact, look at their very burning camp and chop down a very tiny group of survivors before ambling over to our portal directly on the right.
…Well, i do think those instructions could use some cleaning.
lets be real the castle is totally undefended there is no single soul there so for me there was never the illusion that breaking the siege would do anything
I always though the show was that the rats breached the first wall but there was a second behind it where fighters (that we cant see) still engage them but they are losing badly.
Then we pop into the middle and cut off the extra rats, a bunch of attackers abandon the defenders and go for us letting the defenders hold on while we decimate everything we spot (catapults included) and thus break the siege.
There’s some holdouts. If you pay attention when you first come up from the cavern and drop into the fort, you’ll notice the tower to your left has shots being fired from it and skaven climbing up as though trying to overwhelm the final redoubt.
Sadly, the bullets fired don’t actually do anything. You can watch them pass through enemy models with no effect (when one is lucky enough to hit; they don’t seem to be aimed at all).
there are bullets being fired, but there are no people.
as i said not enough to create the illusion