Alright, so I've recently been playing a campaign where I get to play a Pact of the genie warlock in a small town frontier setting. I am the kind of player who likes to have large impacts on the Economy and political dynamics of the world, so when I read that the Dao offered warlocks "wall of stone" as an addition to their spell list, I immediately went to work -- getting a permanent spell effect 3 times per short rest has got to have some potential to be crazy powerful.

So, I created a floorplan for a 2 story structure using the ten 10 x 10 panels allowed by each spell, and a 50ft by 50ft 2 story structure with room for 15 bedrooms, a shop, a living room, a foundation, and a ceiling is possible with 160 panels. i made every wall in the interior of the design 6inches instead of 3 like it is possible for the spell for structural integrity reseason, but if you want to squeeze more out of this you can by making 20 10x10 panels for the interior walls.

The house has some characteristics that are important to note -- it is a little less than 5000 square feet, has enough bedrooms for multiple families, and the bottom story has a place for commerce and crafts. In addition, the second story is stackable onto itself, so you could make a few stackings of this if you wanted to, I would wager about 4 is the max for a construction made entirely of solid stone.

Here is the fun part --

Let's assume that you have a level 11 Dao warlock, (though this is technically possible with a 9th level warlock, you get your 3rd spell slot at this level so the process is much faster) and that this warlock is hell-bent on making a city, so they spend all 16 waking hours building this city. He wakes up able to cast 3 walls of stones, and each casting takes 10 minutes to make a permanent wall, and it takes 1 hour to recharge those uses, so that is a 1.5 hour casting period for 30 panels, with the exception of the initial casting of the day, which is only 0.5 hours as the warlock is fresh from a long rest. 16/1.5 = 10.66, or ten with a remainder of 1 hour. subtract the 30 minutes from the initial casting, and you have 30 minutes of free time a day, and 11 casting periods of 3 uses of "wall of stone" per day, or 33 castings. In construction volume terms, this amounts to 330 panels per day, or a little more than 2 multi-family shared apartments per day.

do this every day for a year, and 2 times 360 (giving the little guy 5 vacation days a year) is 720 structures. Each structure has 13 bedrooms. I am going to consider about half bedrooms shared for the setting I am playing in, but adjust this for your world, so that amounts to infrastructure for shelter and commerce for about 20 people per building. 720 times 20 equals the facilities to provide 14,400 people a place to live.

14 thousand people, given high quality and durable shelter in a single year by a single (albeit dedicated) person. The estimated size of the average medieval city is about 10,000 people.

Here is the link to my blueprint:
https://imgur.com/lhWZytr

The black lines represent the foundation tiles, and the red lines represent the walls. the blue writing is just my designation for rooms, but you could use them however. Feel free to poke holes in this as necessary, I am always open to critique. I am excited to see what you all think about this.