Richard and Tim,
Thank you for your quick answer.
All you can do is make a contract that the owner moves far away, so that the fortress is not protected, and so that you can conquer it easily. But you still have to move through the fortress.
Yes, of course, but this doesn't apply to allied powers. There's no way for Prussia to offer (transfer) a Prussian controlled fortress to the French.
Example: the Saxon army has captured Eger which now bears a Prussian flag. Saxony becomes neutral and Prussian troops are campaigning far away from Eger. France cannot protect the fortress which is now immune to any French action until it is recaptured by the Austrians.