The following points are important to bear in mind:
Search Engine Optimisation is an ongoing task, the 'rules' used change frequently and simply undertaking SEO work once will not guarantee you a high ranking for the rest of time. Unique content is important, but the user experience - the ability of a user to actually interact with your site and find what they need regardless of how or where they are accessing your site from - is becoming far more relevant. If a search engine finds it difficult to navigate your site (e.g. needs 7 'clicks' to reach an article) it will assume that real users will encounter similar difficulties, and you may not rank as well in search engines against competitors who have a better user experience.
Sitemaps can be helpful to provide search 'bots' with a list of pages on your site, but this does not negate the need for an easy to use, clear navigation structure.
Although Search Engine Optimisation is important, focusing on the basic elements of the user experience (easy navigation paths, unique and compelling content etc.) is often one of the best ways to ensure a higher ranking. Simple steps like ensuring appropriate Meta Keywords and Internal links may help to improve that experience further.