private boolean isAreaEmpty(int x1, int x2, int y1, int y2)
    {
        if (x1 < 0 || y1 < 0 || x2 >= dungeon.length || y2 >= dungeon[0].length)
            return false;
        if (!isHorizontalLineEmpty(y1, x1, x2))
            return false;
        if (!isHorizontalLineEmpty(y2, x1, x2))
            return false;
        if (!isVerticalLineEmpty(x1, y1, y2))
            return false;
        if (!isVerticalLineEmpty(x2, y1, y2))
            return false;
        return true;
    }