Miyazaki's very first film is also his least seen. Since it has no connection to Studio Ghibli, I've noticed people sometimes completely leave it out when discussing the Miyazaki canon. However, don't let that deter you as THE CASTLE OF CAGLIOSTRO is actually a great time at the movies!
What's interesting about this film is that it starts out so-so, with weak characterization, cartoony slapstick, a generic damsel in distress surrounded by bland male characters, and a plot that's campier than camp. However, the third act is surprisingly strong and Miyazakian. We get political satire, an exciting climax (that THE GREAT MOUSE DETECTIVE totally ripped off), a mythological element, and genuine emotion! It starts out as a generic movie and becomes a Miyazaki movie as it goes on.
Basically this film is the textbook example of a director developing his craft and finding his voice, and you get to watch it happen right in front of you!