by mvl4d
You must attack the stronghold. Any units' abilities can be used during this attack(Fight goes as usual only that the defender draws no fate cards because of not having any units in the area).For example, the Siege Weapons of the Daqan Lords have the ability to reduce the strength of a stronghold (if I'm not mistaking)
Notice that the Stronghold has it's defending strength, you must have more units than the strength of your stronghold. If it were a "capital"(banners of war expansion) it would've had 7 strength so 6 units would not have been enough for you to cross the area.
Enjoy.