I love this movie. Love. Love. Love. The storyline is imaginative yet realistic, and the animation is stellar, especially for an anime. I got a lot of insparation from this (Animation and art-wise...I love drawing and animating shorts!! <3), but it is rather violent.
SPOILER ALERT: One scene (Hahahah...and one of my favorites), it's the middle of the night, and Howl is walking in to his Moving Castle, apperently after a long and exhausting battle. As he stumbles up the stairs, a trail of think, red blood can be seen. He goes into his room upstairs, and the door shutting wakes up the main charachter. She follows him, trying to avoid the blood, to make sure he's okay (odviously not). When she opens the door, Howl has turned into some type of bird-like monster, with feathers and blood covering the room. He's sitting up in his bed, breathing heavily and gasping. She reaches out to him, but he only says, "Go away." After this motion and line are repeated a few times, he--or should I say it--turns on her and rampages. She gets out in time, and in the morning Howl has returned to normal. No blood, no feathers, nothin'.
END OF SPOILERS
...Just thought ya might like to know. If that were live-action, I don't think the movie would've been rated PG.
Other than that AWESOME scene, the rest of the movie is great. So, if you're watching it with young children, I suggest you just skip it. :)