After reuniting with Ryoma, you lead the resistance fighters to Nohr.
Defeat the Boss
You start the battle south of the castle, and there are a number Nohrian units to defeat.
Charlotte and Benny are standing at the entrance of the castle. You have to defeat them to get in.
More enemies will appear inside the castle after a while.
After you defeat Benny and Charlotte, you should use the Dragon Vein that they were standing on. This causes a landslide in the castle, damaging all the enemies inside for 10 HP each. There are also two more Dragon Veins just inside the entrance, both of which also cause landslides. Your units will not be damaged by the landslide, so you should definitely use these two Dragon Veins inside the castle. This will reduce the HP of the castle enemies by 30, leaving some of the enemies with only 1 HP.
Once you have entered the castle, take your time fighting the enemies inside. Even though they have had their HP drastically reduced, some of them are still tough to defeat.
After the Battle
An important event will happen after the end of the next battle. You can change the course of events, but you will need to do so in your castle before starting the next battle. If you want to know more details and don't mind spoilers, read the information in Chapter 15.