base directories / URL

You can manually create pages named category and app in admin under the Pages menu, and use a custom page template for each to list out taxonomy terms or whatever you need.

EDIT – 301 redirect a request that matches the pagename rewrite to another page:

function wpa_parse_query( $query ){
    if( isset( $query->query_vars['pagename'] )
        && 'apps' == $query->query_vars['pagename'] ){
            wp_redirect( home_url( '/other-page/' ), 301 );
            exit;
    }
}
add_action( 'parse_query', 'wpa_parse_query' );