In that case...

"Case" is a word with a lot of different meanings, one of which is "container" - so we have briefcase, jewel case, suitcase and so on.  In the early days of printing, individual letters were stored in large drawers known as cases and for whatever reason, it became accepted that the capital letters would be kept in the top drawer, and the ordinary ones in the bottom drawer - hence upper-case and lower-case. Now there's interesting!