Actually, directory depth has little to do with anything anymore. It is more about how many clicks from the home page and/or primary topic page any page is.
Taking SEO for example, let's say you have a link in your navigation on every page to the SEO primary topic page. This page would be important for three reasons: one, it is linked from the home page; two, it is linked from every page; and three, it is within the navigation. It is important to use the
nav tag and use nav or navigation for the class name for the div tag to properly signal your navigation. From there any page that is linked from the SEO primary topic page is seen as important, but pages more than one link away will lose importance as it goes. In this case, you can draw a cloud around the SEO primary topic page and any other SEO sub-topic page and indeed do this for any topic you have.
In this way, it is important to gauge a pages importance by number of links from the home page and from any important primary topic page. The further away it is from either of these, the less important the page is. Think of the PageRank schema when mapping your pages. Each page that is further away loses any rank that is passed through the links.
You can sculpt your site using this method. However, I would not try and be too manipulative. Be natural when signaling importance. In this way, you can signal importance for any page by changing how you link to it. As well, you can use any other navigation available such as side-bar navigation to signal importance. Just know that navigation blocks are seen in importance in the order they appear in your HTML code. So your header navigation should be first, any side-bar navigation second, and footer navigation last.
Also keep in mind, importance can come from cross-linking between pages. Do not be excessive and inappropriate, but topical. However, you should be able to adjust a pages importance this way as well. It is important to cross-link between pages as much as you can without getting silly. Think of the user first. But also think of the signal you are sending.
Lastly, I do not have to tell you that on-page SEO signals are important too. People can confuse poor performance in search with factors that may not be a problem, but rather that the page just does not perform as well as it could using on-page signals, link signals (off-page), and of course inbound (back) linking signals. All of these things add up to gauging a pages importance.