Internal linking and error pages
Internal links not only provide additional context to the given topics but also help with navigating the robot around the web. For example, a page that is being frequently linked within the website (e.g. from the main menu), will be more important in the eyes of the crawler than other, less linked pages.
It is also important to ensure that every page is part of the internal linking structure in order to avoid so-called orphan pages. Moreover, linking to a non-existing 404 (not found) pages is also generally an obstacle for the crawler which should be fixed.