This is the first defense mission of considerable difficulty - you will be surrounded.
You begin the mission on a hill, surrounded by enemies (the red arrows). Your task is to survive, even though there's going to be a lot of enemies. To make things even more difficult, the place will not be too convenient to defend. Your main advantage will be the few reinforced positions and three catapults. On normal difficulty, you should just summon additional Battle Mages. They will hold the enemies and expose them to attack of your remaining units. The chest on the southeast contains one Fire Rain scroll, which can be useful during a crisis.
Playing the mission on higher difficulty (orange arrows on the map above) will require you to face even twice as many enemies from all directions. The greatest forces will approach from the north and east. You should have at least one Mage and a few catapults/crossbowmen. At least one unit of cavalry will make your defenses more flexible - you can deploy them wherever they're needed most.