Building your own NPC city is like raising your own barbarian pet. You get to decide what level NPC city you want!
The NPC city you get will be based on the level of the valley you conquer. If you want a Level 5 NPC city, take over a level 5 valley.
- Conquer the desired level valley
- Build a second city
- Build a rally point
- Move all of your 250 workers back to your main city with as many resources as you can carry
- Abandon the city (go to your main city, click Overview, open Cities tab)