Win condition: Rout the enemy
As Lukas points out, there's an archer with a Steel Bow toward the right side of the castle, with a dangerous vantage point to target your units if you get too close. Stave off initial enemy forces, then take out the archer using your own archers or mages. If you don't have any, use Silque's Nosferatu, and then send a flying unit over the wall to finish the job. Make sure it's a sure defeat, as a miss could be deadly on the following turn. Equipping said flying unit with a leather shield ought to help.
From there, it should be easy to dispatch the leader and the remaining enemies how you see fit. Whichever unit you've assigned the leather shield to is a good choice to take the leader head-on, as he regenerates health each turn.
Defeat the enemy commander to clear the map.
