What is the source of the Mages’ power?

Okay, thanks for the answer. I just have a couple more questions. First, what does it mean to "fight with them in 4 stacks"? Next, as I understand it, you should upgrade the mages first, but I didn't quite understand the purpose. Then, who should be upgraded next? Goblins? I've heard they become quite strong. So, maybe upgrade them in the early stages? I also have some questions about the creatures. Djinn - I don't like them at all; they die like flies, and their ability isn't that great either. I build them very late in the game. Colossi - I try to keep them in the castle; otherwise, even 6-7 colossi don't have much of an impact, and they only become effective when there are many of them.