Sorry Doctor, but I, too bring tales of lands other than Sapience.
As a kid, I left the streets of Ashtan to escape certain people who wanted my penial glands. I left by stowing away on a ship... which happened to be pirates, and spent 6 years plundering lands in the far west, and returning to Ashtan to sell the goods. So, undoubtably there is more to Achaea then Sapience.

Mind you, being a Sentinel, and quite used to the view from an Eagles eyes even from atop the mighty Vashnars, I can also say this:
Dont give up your theory just yet, as there is also the possibility that even the mighty eagles cannot see an infinite distance, in which case we may not be able to see the neighbouring continents not because Achaea is round, rather because we simply cannot see this far.
Or, it could be just my eyes, not everyone eyes.
I would recommend you seek out Blackleaf, who has spent many years sailing the high seas, and Covenant, who was born overseas.
Well, I look forward to hearing of the final answer.
Caled

Penned by my hand on the 18th of Scarlatan, in the year 245 AF.