With the recent Google Algorithm update, it is not easier to rank sub pages even if there are no prior external linking to the sub pages. You can build links to your home page and as long as the home page has internal links to your sub pages Google will crawl and find the most relevant page for that particular keyword that you are targeting. The URL seems to have a big impact on this decision. We have done case studies in many different websites. For example our own website vizionseo.com. We are rank on 1st page for keyword “local search SEO” for the sub page url: http://localhost/demo/local-search-seo.html. The reason being is because we have internal link from home page with anchor text that links to this page. Even though link building is only being done on the home page we are still able to rank for this sub page on 1st page Google.
To summarize things up. It is very important to have SEO friendly urls targeting your keywords with proper content of 2-300 words minimum. This will allow you to rank for many keywords besides just what the ones you are targeting on your homepage. Each page can really only be optimize for 3-5 keywords. Although more relevant keywords will rank as you do link building.
