I don't quite understand how to fight there.

It should be quite simple. The guys from Ubisoft simplified the navigation using drag-and-drop technology. Basically, you drag the unit you want to attack onto the unit you want to attack. You can make 5 actions in one turn. The same applies to editing your army; drag the unit you want to recruit into your army (you can have a total of three units), and then specify the number of units. After that, on the battlefield, you choose your opponent and start the battle.