Clarity is the most important keyword in creating navigation solution for a website. Sometimes clients ask for original design, which doesn't express explicitly how to navigate through the website. It is a matter of choice whether to prefer originality or more traditional approach but, usually, users are not keen to re-learn core navigational principles on each page they see for the first time.
Using horizontal menu in the header of a website is a sure thing. Even better would be to use dynamic horizontal menu, which displays active links and surrounding contextual content in the sidebar.
Users understand overall page structure better, when there is one main navigation menu, which contains clear sections like "Articles", "Documents", "Manuals", "Contacts" etc. There may be subcategories under each of them, that's absolutely fine.
What is important about three or four level subcategories, that it is advisable to show which categories are active on each page while navigating. It supports two overlapping goals: 1) Users see where they come and are able to memorize the path to particular page. 2) Users also will have sense of security - they feel better when seeing clearly where they are in the overall surrounding context.