I would say, Boergar showed us mercy, having witnessed our courage and determination, and let us leave the city unscathed, secured in the knowledge that having beaten us that day he will be able to do it again should we decide to try and retake Pranzis (well, he will see what we can do).
Or
We escaped through the backdoor down the secret passage under the castle.
//there is always a secret passage exiting the royal castles.