Why is flat essential?
Flat site architecture is essential for SEO, but also users and their ease of use. With each level of depth, you lose some link juice, PageRank algorithm, and usability. Therefore, by keeping the site flat, you will increase usability. You will also get more link juice in the search engine indices.
How to build flatness
Begin with your homepage with mR/PR around 7.5. From here, link to a site map and other categories. From these pages, you can link to further depth if you need it.
Consider cross linking
Use opportunities to link to other sub-categories to increase link juice.