It's a difficult scenario where Mercia is surrounded by her inner darkness. Mercia must survive for long enough so that her reinforcements can arrive and assist in the fight against Dark Mercia.
The player needs to divide their gold between healing Mercia's defending units and recruiting new units to send as reinforcements.
Beware of Dark Mercia's trebuchets!
There is a cutscene at the start and a cutscene when Dark Mercia is defeated.
(my Wargroove username is caparo)