Win condition: Rout the enemy
The music changed, so you know this must be an epic fight!
The door to the left is locked. To unlock it, you can send flying units over the wall and approach it from behind, pressing "door" from the menu. Once opened, send your army in, leading with powerful or defensive units. Archers to the left side of the castle can be taken out with melee units, while archers above to the right can be confronted directly with flying units (make sure they can dispose of archers in a single turn or take enough hits to get them the next turn) or assaulted by approaching from behind using the staircase.
With dangerous units eliminated, begin storming up the middle of the map. If you have Knights or Paladins, clog the middle entrance with them and Alm, absorbing hits and allowing your archers, Silque, or Lightning Sword wielders to pepper enemy units who get too close. Slayde is formidable but not overwhelmingly deadly – the best way to eliminate him is using Nosferatu, but continuous attacks from Alm or another strong melee unit will do the trick as well.
With Slayde and his army cleared out, Desaix will flee – proceed to the back of the map. First you'll want to take out the enemy mage: if you have a knight who can withstand a magic attack, you can use him to take out this unit then easily withstand the attacks from surrounding enemies. If not, you can send in a flying unit holding a defensive item, or eliminate the archers on either side first from behind the walls, going after the mage from there.
Finally, take out the enemy Knight to complete the map.