Doorway pages are a very bad idea for several reasons, though many SEO firms use them routinely and some businesses use them “by accident”.
As a rule of thumb, if you can't reach the page by following the site navigation, then it is a doorway page. You are not supposed to "visit" the page. Instead, you are just supposed to find it in the search results and then click through to get to the site in question. In essence, a doorway page is no more than a one-page click-through advertisement for a website.
However, when you are searching, you don't want to visit one-page click-through advertisements for a website. You want to visit websites. Think of doorway pages as giant banner ads, only worse. You're searching for cleaners" and instead of getting a cleaners site, you get a page that says, "click here for cleaners." It’s for this reason that Doorway pages are despised by Google.
The fact is that doorway pages don't do a very good job of generating traffic, even when they are done by "experts." Many users simply hit their back buttons when presented with a doorway page. Still, many SEO firms count those first visits and report them to their clients as successes. SEO Firm: "You got 140 visits to your site today!" Client: "But no one visited our products pages!"
I have copied this snippet from Google’s webmaster guidelines:
If an SEO creates deceptive or misleading content on your behalf, such as doorway pages or "throwaway" domains, your site could be removed entirely from Google's index
The full section can be found here:
"My advice would be to ask anyone that has a doorway page (or even anything that comes close to it) that links out to your site to remove the link immediately." said David Clarke of DBS.
If Google removes your website from its index it means you will not appear in search engine results. Full stop. Game over.