A good, well built site with a flat link architecture (or as close to this as possible) is an important start to any SEO campaign. Other factors that make up good on-site SEO would be clean or human readable URLs, congruent content from the top of the page to the bottom of every page including page title, meta description and H1, H2 and H3 tags.
The links into each page need to match the content and links to other internal pages need to have a similar theme expanding relevance and link authority to other pages within the website. So the entire site needs to have a top level theme and then chunk down into smaller more specific pieces. With all of this in mind it is always my goal to have any page accessible in 3 to 4 clicks or less as this helps with site usability and SEO.
Based in the beautiful Heritage Town of Killaloe/Ballina, I'm an