I really liked the detail you put into the Death Knight story. Kudos on getting to the "big time"! I hope to one day, make my own mark.
Anyway, I only have one problem with the keep: it has no anti-magic defenses. A party could easily go in and teleport the guy out then teleport in, looking for their target. 15th level has a plethora of options to choose from, especially with spells like discern location and scrying.
Otherwise, it looks pretty sound.