The simplest piece of advice I ever read about creating a website / optimising a website was years ago and it is still useful today:

make sure all your pages link to at least one other in your site

This advice is still sound today and the most important piece of advice out there in my opinion.

Check your pages for broken links. Seriously, broken links are a waste of link power and could hurt your site, drastically in some cases.

Google is a link-based search engine – if your links are broken and your site is chock full of 404s you might not be at the races.

Here’s the second best piece of advice, in my opinion, seeing as we are just about talking about website architecture;

link to your important pages often internally, with varying anchor text in the navigation and in page text content

Especially if you do not have a lot of Pagerank.