According to the encyclopaedia entry on the Great Fall, Craymen wiped out “all the satellite regions of the Empire”.
thewilloftheancients.com/res … great-fall
All of them? I wonder just how many there were. And whether he wiped them out with explosives like the capital, or lured monsters into the Imperial towns/cities like he did with Cainus. Perhaps he timed multiple bombs to go off at once, however I don’t think the other attacks are mentioned anywhere in PDS.
He probably wiped out the more defenceless foothold areas. It seems a bit vague to me. Maybe it’s meant to be left to our imagination, but I doubt that Craymen could do much more than slow down the Empire.
He found a severely devastating weapon to use on the capital, so its not unbelievable that he found more of them to use on other Imperial cities. He would have wanted to wipe as out as much of the Imperial airforce as possible; multiple city demolitions would cause that kind of efficiency. It would also help explain why we see very few classic Imperial ships in Orta.
The explosion came from the sea, so it makes me wonder what exactly it was. That was certainly no ordinary weapon. But Craymen did take the Empire off guard, that’s for sure.
There’s enough hint of Craymen’s tactics from Azel, I would guess he saved his biggest and best surprise just for the capital, and it likely took a long time to set up. But he had control of those two fleets to raid, debilitate and generally sow chaos in the weaker outposts so that the Empire could not easily respond and intercept him from gaining to the Tower of Uru once he found out where it was. And yes it was probably as much to delay intelligence channels back to the Emperor as much as anything.
I’ll just imagine the tactics and methods were quite varied and dictated by expedience. 
They were certainly organized rebels, but they weren’t going to defeat the Empire by playing by its rules.
We have to give credit where credit is due. Craymen certainly knew what he was doing, but in the end all he could do was delay the inevitable. Still, I wonder how much damage Azel and Atolm alone could have done if Edge’s dragon hadn’t stood between Craymen and his dark dream.