Typically, they're considered Power Metal (bardic, epic-fantasy/europagan themes(dragons, wizards, elves, dwarves, trolls, dungeons etc), much more melodic and happy-sounding than other metal types). I'd say they're a bit more aggressive and melancholic than that.
If you wish to check them out, my personal recommendations are:
"Mirror Mirror" (the first one I checked out)
"Valhalla" (preferably this live version: https://m.youtube.com/watch?v=CZ_VFJn2kJM)
"The Bard's Song (In the Forest) amd (The Hobbit)"
"Wheel of Time"
"Time What is Time"
If it's the albums you're interested in, I'd choose what I called the three Silmarils of Blind Guardian, namely "Imaginations from the Other Side", "Nightfall in Middle-Earth" and "A Night at the Opera"