Díky pristupovať som chcel na piecesOnBoard, ale zabudol som dopsať

public PlayBoard()
        {
            InitializeComponent();
        }

        private void PlayBoard_Paint(object sender, PaintEventArgs e)
        {
            DrawBoard(e.Graphics);
           DrawPieces(e.Graphics); // <-----------------------------------------------------------------------------------
        }

        private void DrawBoard(Graphics g)
        {
            for (int i = 0; i <= boardSize; i++)
            {
                g.DrawLine(PenGrid, 0, i * fieldSize, boardSize * fieldSize, i * fieldSize);
                g.DrawLine(PenGrid, i * fieldSize, 0, i * fieldSize, boardSize * fieldSize);
            }
        }