In reference to chesterfilms review below, this film should not necessarily be viewed as a part of Miyazaki's main body of work, but is better seen as an interestingly divergent piece of the larger Lupin the Third series.
For those uninitiated, Lupin the Third is one of the best-loved anime/manga characters of the last 40 years. He's a crime-fighting master criminal who specializes in daring heists and spy games, along with a little bit of girl/world-saving action. His adventures typically take equal inspiration from James Bond and Bugs Bunny and put him up against outlandish supervillains, as well as his pseudo-nemesis Inspector Zenigata, a hapless detective trying to bring Lupin to justice. Lupin is assisted by his three main accomplices, Jigen (a slouch-hatted, constantly-smoking gunman), Goemon (a stoic samurai with a nigh-unbreakable sword), and Fujiko (a sexy femme fatale who shamelessly leads our lecherous hero on), who supply cool gun-fu action and Marx Brothers-like comic relief.
Unlike the majority of films and TV episodes in the Lupin series, which present the main character as an restless go-getter given over almost entirely to impulse (not to mention trying to get busy with every woman he meets), Castle of Cagliostro presents a somewhat more sedate, mature Lupin. Miyazaki's lighter touch makes Lupin into more of a charming, romantic hero and less of a big bad wolf, and the entire film has a somewhat gentler feel than the other Lupin films, which may make it a bit more accessible for folks who haven't seen the rest of the series.
This is not to say that Cagliostro isn't still plenty exciting for the red-meat action movie connoisseur. There's a harrowing car chase sequence towards the beginning that perfectly balances cartoon physics with a stylized realism, as well as an absolutely astonishing final battle in a clock tower that has been relentlessly homaged/ripped off in dozens of animated films since. Although its low frame-rate 1970s animation seems a bit dated today, the film itself is a timeless classic and is as enjoyable today as it was when it was released.