[PvK]

I think you usually get two battles:

One battle outside the castle (no castle on the battlefield), between the arriving enemy relief force combined with enemy units trying to break the seige from within, against any units you have in the province who are NOT storming the castle (and who aren't hiding).

Another battle at the castle, with the defenders left inside the castle, and the troops you have ordered to storm the castle.

If you lose the first (exterior) battle, but win the interior battle (for example, if you send most or all of your forces to storm the castle), I think I have seen that what happens is you end up with your army inside the castle, but the relieving enemy army outside the castle. I also think the defense value of the fort gets restored at this point (which isn't unreasonable, if you imagine that the former attackers, once in control of the fort, are able to dismantle their siege towers, collapse their tunnels, set up their seige engines for defense, etc.

PvK