Who "owns" a province with besieged fort?

[Huntsman]

For purposes of determining sequence of orders, who owns a province that has a fort under siege? The sieger or the defenders?

Specifically, I'd like to know the order of the following events:

1) I'm sieging a fort and I set my orders to "Storm Castle".
2) The defending player sets orders for an army in an adjacent province to move into the besieged province.

Who moves first? Do I storm the castle and fight the defenders *before* the other player's army attacks?

Thanks!