What ive tried so far....
Just at night go to teh closest village... go to the elder and talk to him... he will give you an option to start a fire... ask him to do it just at midnight.
Go to the castle and wait for midnight... enter the castle...
If you are at war with the faction it will be very hard since you will sneak in with a stick and some robes... and they can (or not) catch you trying to go in...
If you are at peace you will enter fully geared.
Talk to the prison guard and ask to see the prisoner then ask for the key...
Attack/kill the guard, enter the prison before the rest of the guards arrive. Talk to the prisoner, and ask him to follow you (even if hes behind bars in a cell)... go outside to teh courtyard... make the prison follow you (F1 F2)... since there is a fire in the nearby village you should face less guards...
KILL THEM ALL
Done!
At least this have worked for me and never got any sort of bugs, altho there is a lot of people reporting bugs in these rescue missions...
Hope it helps...
