this:

     if (_assets.Warriors - card.Cost.Warriors >= 0
         && _assets.Mages - card.Cost.Mages >= 0
         && _assets.Kings - card.Cost.Kings >= 0
         && _assets.Health - card.Cost.Health >= 0)

Has no business in the `PlayerBase` class. In as much as `GameCard` contains both of these objects, this logic should be there as well. And of course it needs to be encapsulated w/ descriptive name.

**Fractal OO**

Go deep w/ abstraction/encapsulation. For example:

     public void Run(IEnumerable<PlayerBase> players)
    {
        while (players.Count(p => !p.IsDead) > 1)
        {
           //redacted
                if (!player.IsDead && players.Count(p => !p.IsDead) == 1)
                { // redacted
                }
            }
        }
    }

could read more abstractly:

    public void Run(IEnumerable<PlayerBase> players)
    {
        while (CombatStillPossible)
        {
           //redacted
                if (LastPlayerStanding)
                { // redacted
                }
            }
        }
    }

**`Run()` is fuzzy**

Looking at the original code as just the method parameters together w/ the overall control logic, it is not at all clear what I'm seeing. Also, the seeming nested-repeated logic is making me wonder about flawed logic or poorly structured logic.

I wonder if you get the outlier conditions out of the way first thing then all the logic is clearer.

     public void Run(IEnumerable<PlayerBase> players)
    {
        if(NoLivingPlayers) { etc, etc, return;}  // covers a null argument too
        if(OnePlayerAlive) { // we have a winner, return;} // covers the case of a collection of 1 player

        while (CombatStillPossible)
        {
            // internal logic would have avoided the "3 players bug"
            // internal logic keeps paring players and fighting each pair
            // until only one is left.

            Tournament meatGrinder = new Tournament(); // pass in all the players
            PlayerBase winner = meatGrinder.Fight();

            // a null winner means everyone is dead
            // "winner" could be class variable so it can be evaluated in
            // NoLivingPlayers, etc.

            if(NoLivingPlayers) {// oh well, etc.; continue;}
            if (LastPlayerStanding)
            { // winner, etc.;}
        }
    }

Sure, the above is putting off actually doing anything, but we're going fractal here. `Tournament` will internally pair players...

    public class Combatants {
        public PlayerBase Player1 { get; protected set; }
        public PlayerBase Player2 { get; protected set; }

        public Combatants(PlayerBase player1, PlayerBase player2) {
        
        }

        public PlayerBase Fight() {} // here is the logic of single combat.
    }

**What We Have**

 1. The game *driving* the tournament
 2. The Tournament *driving* the pairing of combatants
 3. The Combatants *doing* the fighting.
 4. Better separation of concerns
 5. We coded at appropriate levels of abstraction w/in each class. A.K.A. we pushed details down.