The steampunk part of the setting comes from a substance called Cavorite, used to construct airships and generally used in miraculous machines. Including one that allows one of the girls to fly and do various other amazing feats. The world at large is at early 20th century levels of technology and culture, but those Cavorite-based machines are highly advanced. All of it is beautifully drawn - the artwork is top-notch, whether drawing steampunk tech or the rest of the world.
Once you get past the usual suspension-of-disbelief about very young characters doing all these crazy things (which is necessary to almost all anime series and almost second nature for me by now), the overall story and individual episode plots in Princess Principal hang together pretty well. I like the world design, and I found the world-building history aspects interesting. Each episode generally follows a single spy mission, always going wrong in some way, as one would expect from this type of premise.
Chronology in the series is a bit odd, with the timeline jumping around with each episode. First you see a mission with all five girls, then we jump back and see how four of them first met, then a few more missions, before finally meeting the fifth member. And so on, until the last few episodes finally wrap things up in order.
For such a short series (twelve episodes), the character design is very good. Each of the five girls has some time devoted to their individual stories. Character growth is a bit odd because of the jumps back and forth in time across the episodes, but you can definitely see changes as time progresses. And of course the story of the princess and her closest friends is a big part of the overall storyline.
I greatly enjoyed Princess Principal and wish it had been longer. I see they're planning a film series next year, which I'll definitely be looking for.
