Keara's was the first flying fortress I did myself

It was slow and careful going, but I got a new level almost every platform. Enjoy the XP while it lasts, by your final fortress, you'll be doing one-hit kills for almost no XP.
And Broken Valley isn't to hard, as long as you advance carefully and don't 'pull' too many enimies at a time. Also, stake out your landing points (there are some safe spots on hills) - you'll want to be able to use your human regeneration and healing powers. I found it slow going even at higher levels, because my dragon form was always so weak compared to my human form.