Newbie Question: Defending Fortresses

[Joseph]

I just recently started playing Dominions 3, and I had a question about fortresses. If I have a large number of units sitting in a province with a fortress, and an enemy invades it, it seems to me that the units there just remain in the fortress and leave the initial defense purely to the units I get from province defense. Province defense of course gets slaughtered, and then I'm left with the option of breaking the seige or staying in the fortress.

What I'd like is the option to have the units in the fortress fight alongside province defense instead of just letting province defense go it alone and then lose my province defense value in the province when they lose. I don't like having to repurchase province defense every time an enemy decides to attack an area with a fortress in it.

Is there something I'm missing here?