tl;dr
Horizontal nav is good for overviews.
Vertical nav is good when you need persistent deep-dive controls.
Landing pages are an immersive experience worthy of experimentation.
E-comm has been around a while now. We have some solid, generalized patterns for shopping experiences.
Top / horizontal nav is a good pattern for exposing high-level categories. It's the first place shoppers will look to get a better understanding of your catalog.
Left / vertical nav can solve two problems: persistent visibility of deeper, local sub-category structure and product filtering controls.
Landing pages are (or can be) unique. These pages are geared toward in-bound traffic of a particular type, often the starting point of carefully architected SEO pathways. A landing page should be engaging for a certain audience and should always be tested heavily.
Case study
Amazon is big, tests constantly, and is familiar to the majority of on-line shoppers.
Their massive catalog uses an unfolding navigation structure worthy of study.
Top nav
At the highest level of the site, they've tested their way into progressively collapsing the top nav in favor of a big fat search box. This works very well for absurdly large catalogs.
Other sites, such as Walmart, followed their lead with what is virtually a direct copy:
These two companies have something few of us ever will: boundless customer volume and financial resources. They are both aggressive with their testing and experimentation.
None of this voyeurism is a substitute for testing in your own context, but it provides a valuable reference point when dealing with large product catalogs: it's all about the search box.
Top with a side
So what might that solution look like if you had a smaller selection. We can still look to our good friend Amazon for a common pattern (compare elsewhere and you'll see similar structures).
If I drill into a sub-department, you'll see the taxonomy begin to expose itself.
Now we can see a horizontal overview with a vertical deep dive. It's a lot to take in, but there's more to the vertical nav to help with that problem:
Second only to the search box, filters are the e-comm power tool of choice. Filtering mechanisms are a key part of the navigation experience and fit nicely into the vertical nav space.
You can see in some cases as you drill down further, the nav component of the vertical space compresses and the filters become the dominant element.
Landing pages
Here again, Amazon has spent a long time working on this problem.
Parent categories, like this one, are usually a good candidate for a lander, though it doesn't have to be a taxonomy-based decision. It's more about shopper entry points (as the name suggests).
The goal is to engage someone at the first point of contact and avoid the dreaded bounce. How you do that will depend on your shoppers, product, competition, and goals.
You can see that Amazon has chosen to create some duplication. Part of that may be a platform constraint (every sub-category gets an expanded horizontal nav, but vertical works better here). The bigger take-away here is the immersive "feature" imagery. You'll find that these landers vary slightly from one category to the next, as each department has a separate responsible team addressing it's own shopper needs.
