Custom filters inside a specific category?

I think these should be separate taxonomies. Category is actually a taxonomy in its own right.

You would create a taxonomy for each of the filters/drop-downs. This may seem like overkill but i think it will make it easier to work on additional features down the road.

Add the following method to your functions.php. Replace ‘rwc’ with your theme’s domain:

function add_taxonomy($singular, $plural, $objects) {
$labels = array(
    'name'                => _x( $plural, 'General Name', 'rwc' ),
    'singular_name'       => _x( $singular, 'Singular Name', 'rwc' ),
    'menu_name'           => __( $plural, 'rwc' ),
    'parent_item_colon'   => __( 'Parent '.$singular.':', 'rwc' ),
    'all_items'           => __( 'All '.$plural, 'rwc' ),
    'view_item'           => __( 'View '.$singular, 'rwc' ),
    'add_new_item'        => __( 'Add New '.$singular, 'rwc' ),
    'add_new'             => __( 'Add New '.$singular, 'rwc' ),
    'edit_item'           => __( 'Edit '.$singular, 'rwc' ),
    'update_item'         => __( 'Update '.$singular, 'rwc' ),
    'search_items'        => __( 'Search '.$plural, 'rwc' ),
    'not_found'           => __( $singular.' Not found', 'rwc' ),
    'not_found_in_trash'  => __( $singular.' Not found in Trash', 'rwc' ),
);             
    $args = array(
      'hierarchical'        => true,
      'labels'              => $labels,
      'show_ui'             => true,
      'show_admin_column'   => true,
      'query_var'           => true,
      'rewrite'             => array('slug' => strtolower($singular))
    );
    register_taxonomy(strtolower($singular), $objects, $args);        
}

Then use it to add taxonomies. Replace ‘post’ with ‘page’ if you are looking to filter pages

function add_custom_taxonomies() {
    $objects = array('post');

    add_taxonomy('Gender', 'Genders', $objects);
    add_taxonomy('Level', 'Levels', $objects);
    // .. and so on
}
add_action( 'init', 'add_custom_taxonomies', 0 );