Here is one way to turn one of your websites liabilities into an asset: make a custom 404 page. The 404 error page you land on when you try to call up a web page that doesn't exist usually looks the same on every website. You can easily customize this page and use it like a site map. This is okay to do as long as you do include the proper error code explaining that the page has moved or no longer exists, after that you can show your visitor a list of the pages that do exist on your site.