I really severely doubt that Ishual was built in any capacity by The Dunyain. We've bandied the idea around (or at least, I have) that Ishual is an abandoned Nonman mansion. From it's name being Ihrimsu to Kellhus remarking how the mansion at Kyudea looked so much like Ishual leads me to conclude there is really no way the Dunyain made Ishual in any capacity, let alone dug out the massive complex that is The Thousand Thousand Halls.
To throw a few questions out there:
1- Do we know the location of the ruins of the Nonman mansion of Siöl, since it is mentioned so frequently? Could Ishual have been built on to of it, assuming the theory that Ishual was built on top of a Nonman mansion?
2- It's probably a stretch, but by reading the words aloud, you could pronounce Ishual and Siöl in a similar way (I-Shial or I-Shiöl vs Shial or Shiöl). I have a strong liking for philology and searching for etymological development of words and pronunciation, so this popped up. Any thoughts?
3- Do we know anything about the original people calling themselves the Dunyain, besides the very first part in TDTCB? How did they know how to get to Ishual, since it was a secret refuge/location created by Anasurimbor Kelmomas?
And a jump-in question since Kyudea is mentioned:
I remember a passage in TTT about a war between the twin cities of Shimeh and Kyudea for control of Amoteu, where Kyudea is destroyed and laid to ruin. This looks like a Samarmas/Kelmomas (or a 0 and 1) parallel to me. How do twins tie into the development of the Dunyain?