Creating a custom URL

Here’s one possible solution: a plugin called Remove Category URL.

It will turn this:
http://www.example.com/category/case-studies/

into this:
http://www.example.com/case-studies/

Create a category called SEO, and another called Case Studies with SEO as its parent.

If you post under the SEO category, the URL will be:
http://www.example.com/seo/

If you post under the SEO/Case Study category, the URL will be:
http://www.example.com/seo/case-studies/

Categories must have unique names, so if you want a Case Studies category under two different categories, you would have to do this:
http://www.example.com/seo/seo-case-studies/
http://www.example.com/design/design-case-studies/

I’ve never used this plugin myself so I can’t vouch for it, but I did a test and it seemed to work as advertised. I did noticed that if you deactivate the plugin, you’ll have to go into Settings > Permalinks and click Save, otherwise all your pages will 404.

And of course it’s always wise to back up your site before making any changes.