Having a logical architecture will help search engines understand how content on your site is related, and allow for the effective flow of PageRank
The robots.txt file allows you to control search engine bots access to content on your site, particularly useful for large websites with a lot of content not meant for indexing
Search engines tend to favour content that loads quickly and responds to interactions, so you should look into reducing the amount of code bloat and large images being used
Marking-up your website content with structured data schema allows for extra information to be pulled into the search results, enhancing click-through rates
Websites can easily get cluttered with dead, removed or redirected pages, and it is important to stay on top of these to ensure good site health