After receiving unexpected help, you navigate a maze-like passage and encounter an unexpected enemy on the way to the castle.
Before the Battle
You might want to have Corrin pair up with a flying unit before starting the battle.
Defeat the Boss
The first thing that you should do in this battle is send Corrin north to talk to either Arthur or Effie. If you do, those two units will become your allies, fighting enemies for you until the end of the battle (unless they die). You only need to talk to one of them to turn both of them into allies. If you don't do this, they will fight against you.
Camilla can repeatedly use three Dragon Veins near her. She uses them every other turn. These Dragon Veins will shoot a fireball down the length of one of the big corridors. This will damage all units in the corridor, but it will also burn up the barrels and crates that are blocking the path, making it slightly less difficult to reach Camilla.
Because of the threat from Camilla's fireballs, you might want to hide out in the alleys between the waterways. The enemies will come toward you whether or not you are in range, so you can stay safe in the alleys and wait for them to come to you.
Many enemies in the map drop items when defeated, so keep an eye on what the enemies are carrying.
When you finally approach Camilla, you will have to deal with Selena and Beruka, who are paired up near her. Once you deal with those two, you can defeat Camilla and end the battle.