On bgg someone asked the following question: Can you attack a Stronghold that you have already secured? (He is talking about his own stronghold!)

I would think not (as there is no rule to be able to do that).

No. There's no way that you can attack your own stronghold. Battle is only conducted against a target that is not controlled, but a stronghold in your play area is controlled by you. You couldn't siege it either as it isn't another player's city or home realm.