public final void addWalls() {
        //Build walls in the wall shape
        int xx = 60, yy = 540;
        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < 4; j++) {
                for (int k = 0; k < 3; k++) {
                    if (!(k == 2 && (j == 1 || j == 2))) {
                        walls.put(walls.size(), new Wall(xx + i * 200 + j * 20, yy + k * 20));
                    }
                }
            }
        }
    }