Pokémon Conquest/Fontaine

In Fontaine you must defeat all enemies to capture the castle, similar to what you did in Ignis. You get 20 Turns to do so. The main terrain types on which you will be playing are grass and water. Grass types are recommended for battle as their moves are super effective on water types plus they take negligible damage from water attacks.

To fight, you must first press the switches to close the dam and stop the flow of water so as to enable the passage of non-water type Pokémon. It is better to not keep Pokémon on the river as this may cause them to receive damage when the switches are opened.

Strategy
A grass type Pokemon like Carnivine can make this battle a piece of cake but even without it it's not too difficult. First you should Close all the dams so that your Pokemon can cross the river. It is easier if you focus your attacks on Oshawott and Panpour and then take care of the rest to win the battle. The only tricky bit is getting your Pokemon across the river. Once you get them across it becomes fairly easy.

After the battle you may get to recruit some warriors that will add some water type Pokémon to your army.

Post Battle
After the battle Mitsunari and his friends will again come and have a chat with you and three new castles will open up for you to battle. They are Violight, Chrysalia and Puigilis.