The Keep still isn’t available on DVD but the good news is that as of early this December I discovered you can now get it on NetFlix, the streaming service for niche movies.
Read the write up, which concludes with:
Fans of Michael Mann, Ian McKellan or 1980’s genre movies need to see The Keep. As a long-unavailable piece of modern movie history it’s one of the most joyous discoveries on Netflix Instant Streaming. It’s not very good, but at least it’s utterly fascinating.