You can simply pass multiple arguments into the middleware using a colon. Use it like: 
Route::group(['middleware' => 'checkUserLevel:some_value_of_level'], function () {
            // my routes
        });
Now, you can have this value inside your $level variable. 
public function handle($request, Closure $next, $level)
{
    $user = Auth::user();
    if ($user->level > $level) {
        return redirect('testUrl');
    }
    return $next($request);
}
This would help. 
Edit: 14 Dec 2018
You can also send multiple variables to middleware. You just need to seperate the values using a comma (,).
Route::group(['middleware' => 'checkUserLevel:some_value_of_level, one_more_value_to_send'], function () {
                    // my routes
});
And you will get the value one_more_value_to_send in the variable after $level in the middleware handler.
public function handle($request, Closure $next, $level, $another_value)
{
   $user = Auth::user();
   if ($user->level > $level) {
      return redirect('testUrl');
   }
   return $next($request);
}
For more details you can refer to: Passing parameters to Middleware in Laravel 5.1