You might check the site
http://www.tt-helden.de/. It contains quite a lot of material about technique, stroke execution, serve and receive game, tactics, etc., both playing as attacker and defender. They even cover the topic playing against defenders. The site is managed by a german coach in collaboration with Ruwen Filus.
As it is commercial, most of it has to be payed for, but you'll find some free videos. They talk a lot though and it is in German, but in my opinion the content is very good.
Another source of information is the old instructional by/with Koji Matsushita "For the beginners of choppers". Even without understanding a word (explanations and subtitles are Japanese) I very much like it and it also covers all aspects you'll need for a defensive game. You'll find it in youtube (
https://www.youtube.com/watch?v=BgboCO1bczg).