The heart of the answer is “beat his system” – even an all-powerful mage can’t directly dominate a continent. As for him personally, I commented on this a year and a half ago:
As for the Floating Palace:
Yep, just tens or hundreds of thousands, depending on how long you reign. It’s a powerful symbol if you manage it.
@11110 , let me get back to you on those big questions – they’re not forgotten, but I can’t just toss off answers to them like I can the other, simpler questions in this post. 
A linear Ward would prevent stuff going through it just fine. It would (obviously) not prevent stuff going around it. From the time the last Border Ward was fired up under Eosphora and her Ennearchs, there’s never been a moment when the Hegemony has not been completely encircled – but by nine great Wards end-to-end, not a single loop.
Pushing out a Ward involves adding a new arc of wall outside it, then doing something to make the Ward recognize the outer arc rather than the inner one as the relevant wall. The force of exclusion then rolls from one to the other, pushing back anything that the Ward rejects in the newly added zone.
That’s to keep that Ward a continuous line. In theory they wouldn’t need to – Horion’s team could have built a wall sticking out perpendicular to the current Ward and Changed it so that instead of running to the next Ward in Wiendrj, the southwestern border ward now ran out into the Xaos lands and stopped there. But that would be…suboptimal.