Get custom post type parent category

Your CPT and your CT init seem to be right, but the way you get the taxonomy terms is incomplete.
If you look in WordPress Codex, the function get_terms(); accepts two parameters:

• $taxonomies
• $args

If you want to get only parent terms you have to specified it in your $args array.

$args = array(
 'parent' => 0 // queries only top level terms of a specify taxonomy or taxonomies ( first argument, rememberer? )
);

So in your case, try this:

<?php

$taxonomy = 'btp_work_category';
$terms = get_terms($taxonomy, array( 'parent' => 0 ) ); // Get all top level terms of a taxonomy

if ( $terms && !is_wp_error( $terms ) ) :
?>
    <ul>
        <?php foreach ( $terms as $term ) { ?>
            <li><a href="https://wordpress.stackexchange.com/questions/172109/<?php echo get_term_link($term->slug, $taxonomy); ?>"><?php echo $term->name; ?></a></li>
        <?php } ?>
    </ul>
<?php endif;?>

Hope it’s helps!