A doorway page can also be the main homepage or landing page of your site, yes - a page that sums up the content of your site, links through to your other or main and domain-name/your-goals-related-content of the site, summarises what you're all about, anything that can immediately tell a visitor to the site (who lands on your homepage) the things they want to know, that they searched for in the first place, and that draws them in to explore the rest of your site, and perhaps, hopefully, even lures them in enought to eventually contact you.
To optimize your doorway or homepage for search engines without seeming spammy, it's a good idea to display a decent amount of text on that homepage, in the main body of the page. Images should have alt tags containing your main targetted keywords; your meta keywords, description and title (in the coding/behind the scenes) should have good relevancy - they should not only contain words that you're targetting, but should also be the words that you have repeated a few times on your page - everything links together - or helps each aspect - you can use only good meta, or use only good keywords in your content - matching them up together is best, as well as of course checking that these words and phrases actually have search volume before you even use them (or edit your content and meta and alt tags and title tags) to match up with words and phrases that people are actually searching for.
If you have a lot of content on your site, and frequently update your site with new content ( if it's a standard html/CSS site that you can't manage yourself, then have your designer add a WordPress blog to your site so that its url will be for eg yoursite.com/yourblogsname ) you don't need to have spammy splash pages/doorway pages all over the Intternet linking to your site. Cliche perhaps, but content is king (and more so when it is correctly optimized for search engines, and more of it is frequently added to your site) - adding content to a blog that is on your site, is the same as adding content to your main site, because that blog is ON your site.)