Level.cs

namespace TowerDefenseGame
{
    class Level
    {
        private readonly Invader[] _invaders;

        public Tower[] Towers { get; set; }

        public Level(Invader[] invaders)
        {
            _invaders = invaders;
        }

        //returns true if the player wins : false
        public bool Play()
        {
            // run until all invaders are neutralized or invader reaches the end of the path
            int remainingInvaders = _invaders.Length;

            while(remainingInvaders > 0)
            {
                foreach(Tower tower in Towers)
                {
                    tower.FireOnInvaders(_invaders);
                }

                remainingInvaders = 0;
                foreach(Invader invader in _invaders)
                {
                    if(invader.IsActive)
                    {
                        invader.Move();

                        if(invader.HasScored)
                        {
                            return false;
                        }

                        remainingInvaders++;
                    }
                }
            }
            return true;

            // each tower
        }
    }
}