Usable navigation for a shop with a long top-level category

I have a shop site in which the client wants to remove a left category navigation which currently is used on the desktop version of the site. This seems mostly to keep up with fashion, and it does seem to be the case that vertical menus are no longer as common - so it seems a reasonable request if users are no longer used to them.

Reading around, the current standard for a shop's navigation seems to be a horizontal menu featuring a small amount of top-level categories, each with a rollover dropdown for sub-categories. A vertical layered navigation may then appear on category and product pages. Please correct me if I am wrong here.

Other navigations I have seen are:

• hamburger menus, which still seem a bad idea on a desktop as you need the user to interact before being able to browse the categories. I assume these are still essentially seen as mobile-only.
• massive multi-column dropdown menus - which I personally find ugly and unusuable, and can vanish off the bottom of the screen if too many categories are added.

My client's site has 27 top-level categories, not all of which have sub-categories.

I feel that the best solution would be if he could re-organsise this with 5 or 6 top-level categories, and more sub-categories.

However, if that is not possible, do I have any better options than the massive menu or the hamburger? Would keeping a vertical menu be considered better than these two options?