A simple question regarding tags and categories

From your comments, you opted for a custom page template. Based on that, you can simply perform a custom query to retrieve posts that belongs to a certain tag and category

I tend to make use of a tax_query using WP_Query when you quering posts from more than one taxonomy. It is more flexible, specially if you need to exclude children from hierarchical taxonomies

You can construct your query as follow

$args = array(
   'post_type' => 'post',
   'tax_query' => array(
      'relation' => 'AND',
      array(
         'taxonomy' => 'category',
         'field'    => 'slug',
         'terms'    => 'SLUG OF THE CAT',
      ),
      array(
         'taxonomy' => 'post_tag',
         'field'    => 'slug',
         'terms'    => 'SLUG OF THE TAG',
      ),
   ),
);
$query = new WP_Query( $args );

You can visit the links provided to learn how to construct your loop using WP_Query