With v16 there were a few changes from previous version within the category browsing page's display of category navigation. The default category navigation page does not display category navigation directly within the page but if the currently browsed category does have subcategories they can be chosen by clicking the plus sign next to the current category within the category tree. See screenshots:

This plus sign appears if there are subcategories…

Click that plus sign and you see the subcategories selectable within the following popup:

If you do not like the "popout subcategory choice" you have other options. Within previous versions the subcategory selection was within the category browsing page by default but this proved problematic for display within mobile screens as that could make the page "forever long" to scroll through just to get to listings. You can still return to that with changes in the the following page template that controls the look on the category browsing page by default:

/main_page/category_page.tpl

The syntax changes look like the following within that template. The first option has the category tree module display below the category navigation:

and displays like so:

You can also exchange the order of the category tree and category navigation modules to get the category tree to display on top like so:

Do note within the above options we turned off the display of the default category tree within the following admin tool setting:

SITE SETUP > BROWSING SETTINGS > Category Tree (Breadcrumb) Location

But you can also leave the default category tree/breadcrumb in place but it will only display below the category navigation you place like so:

If you anticipate heavy use of your site through mobile devices you may create a "mobile specific template set" (or at least a "mobile" category_page.tpl within your mobile template set) that does not have the full category navigation module within it…and uses the default display. This makes that page much easier to navigate on mobile devices.