Imagine a city on treads; if you've seen or read Mortal Engines, imagine that instead.
beep beep losers here I come
They're constantly moving, and have decent populations. Don't worry about how it works, or anything like that, just know that they are major trade centers and need to be visited on a regular basis, similar to port cities like New York, London, Shanghai, etc.
Because of this, mapmakers would need to locate these cities on maps. While magic does exist, and it is the basis for the moving cities, I would prefer that these maps be static, regular, maps.
Late renaissance to early industrial technology.
Some more info:
- I would rather not have these cities move on a regular route, and instead I would prefer them to move on the spur of the moment.
- The cities are limited to an area about the size of Mexico, with a vaguely trapezoidal shape. This area is grassland with very little change in elevation.
- The cities move at slow-traffic speeds, at about 10 mph.
- Instant communications, akin to the early internet but with only text, exist; see this question
- More info added on request.
- That "magic communication" isn't universal; it's more like a private net, with only those who are privileged and wealthy being able to receive or send.
Not really sure what tech level to tag it as. Feel free to edit if you think it's needed.