class Tile
{
    TileType Type;
    EntryCost Cost;
}

class Unit
{
    Vector Position;
    float Health;

    // virtual Unit Clone() { ... } ?
}
class Spider
{
    Unit Target;

    // override Unit Clone() { ... } ?
}

class Unit
{
    Vector2 Position;
    float Health;

    public virtual Unit Clone()
    {
        return new Unit() { Position = this.Position, Health = this.Health };
    }
}
class Spider : Unit
{
    Unit Target;

    public override Unit Clone()
    {
        var clone = (Spider)base.Clone();
        clone.Target = this.Target;
        return clone;
    }
}