With as much as 26 episodes the series gets a long time to adapt the book in a fair way, which it does from beginning to end, with some extra fluff that doesn't take away anything from the experience. If you would compare it to the old swedish movie from the 80's, this is definitely the more true to the book. This is because all the relations, events and characters gets the time they deserve to be built up properly. In the movie they basically just piled up all the key scenes as tightly packed as possible, without giving any emotional connection to anything. This is where Sanzoku no Musume Ronja succeeds splendidly.
However, something they did not succeed in was that special feeling of norse fairy tale creatures. The interpretation of the rumphobs (rumpnissar) is pretty awful, both personality-wise and appaerance-wise. The wild harpies (vildvittror) maybe would have worked in a more detailed 2D-animated style, but now they look too flat and cheap. A good equivalent to their classic catchphrase, "Nu ska blodet rinna!" (literally: "now the blood shall flow") is also missing. The gray dwarves (grådvärgar) and the unearthly ones (de underjoriska) were okay though.
Something that the series managed, that neither the old movie nor the book did, was to give all the robbers in Matt's Fort their own distinct personalites. I greatly appreciated this. And also, the interpretation of Skalle-Per was terrific.
Besides some dumb mistakes, like the fact that they pick blueberries and raspberries in spring (they don't become ripe until late summer), the swedish nature is depicted well. Sometimes the tempo maybe was too slow, but this helped more than becoming a hindrance. The thing I missed most from the movie was all the beautiful accapella harmony songs.
This is most definitely the best work of Goro Miyazaki so far. But then he also got really good source material from the great Astrid Lindgren! A fantastic story that always is relevant to people in all ages. Strongly recommended to everyone, give it a fair chance!
