I'm almost sure the difference between "normal" and "casual" isn't that big !
I even find that the diffenrece between nightmare and casual isn't big enough ...
And if you've problems with the Flying Fortresses (dragon combat or the human combat?), ask on this forum for help and I'm 100 % sure at least 2 or 3 people wish to help you giving hints and tips how to beat the enemies !
Can you remember which kind of enemies give you the most trouble ? (Rangers, Warriors, Necromancers, Mages, Healers, ... , Flying Demon's, Balista's, Lightning Towers, Nests, ...? )