Defining same ADT in C# and Scala

ADT.cs

public abstract class Kind {
  public class Global : Kind {}

  public class World : Kind {
    public readonly int world;

    public World(int world) {
      this.world = world;
    }
  }
  
  public class Level : Kind {
    public readonly int world;
    public readonly int level;
    public readonly bool bonus;
    
    public Level(int world, int level, bool bonus) {
      this.world = world;
      this.level = level;
      this.bonus = bonus;
    }
  }
}

ADT.scala

sealed trait Kind
object Kind {
  case object Global extends Kind
  case class World(world: Int) extends Kind
  case class Level(world: Int, level: Int, bonus: Boolean) extends Kind
}

I don't know Scala, so I might not understand well enough what exactly are you trying to achieve. However%

• Why do these classes have to be nested? I've almost never seen a child class as a nested class of the parent in C#.
• If all your classes are immutable, wouldn't you be better off using structs instead?