"Alright everyone, let's talk about the specific matters of subspace planning!"
The Ashen Messenger no longer paid attention to the final battle of the mortals, but discussed with the heroes and mages of the subspace how to construct the first subspace city. Clearly, due to the special nature of this secondary plane, conventional building materials couldn't be used here. Even the Collapse God would have remarked that this place was a wasteland.
Luckily, the Ashen Messenger had just seen some great items on the product page of the Fictional Supermarket!
There was something called "Bedrock" which could be placed and used to construct buildings normally, whether in the void or subspace. It was available for sale in both Block World and Stardew Valley, and a massive vein of bedrock was said to have been discovered in Stardew Valley.