It took me awhile to finally beat this level. My question was (because I'm a completionist), has anyone here (or is there any way) to take all enemy castles on the map before taking on the dragons to win the campaign? I'm aware it might take sometime with their constant onslaught but if I maintain my one castle with dragons and defend off do you think its possible or is this map created just to rush and take the Dragon City?

It can be solved, the main goal of the mission is to occupy the dragon city until then I could do anything in the whole map, i even collected the artifact which can be useful in capturing the dragon city. I gave my strongest hero some dragons, and in a castle built and occupied by AI, he also learned the highest level of spells, such as. the Dimension Door and Armageddon with which my hero became unstoppable only needed a few dragons on his team.