How do vassal cities develop from their tier level?

[Mourioche]

Some vassal cities sometimes increase their levels. I often see them go from T1 to T2 on their own, and more rarely they go up to T3.
Does anyone understand how they do this?
Can we help them? (I don't think so).

 

[Astronlmo]

You can't help them but they upgrade automatically with time.

 

[Wyrd Deofol]

You can't help them but they upgrade automatically with time.

Apart from granting them units, i don't think you can support them economically apart from trading perhaps.

 

[Mourioche]

Thank, yep...
But has anyone figured out how they increase their tier level?

 

[Admiral Boysen]

Probably the same way any Godir's cities do, by constructing town hall upgrades

 

[Blanch Warren]

Apart from granting them units, i don't think you can support them economically apart from trading perhaps.

Vassals (or free cities) don't have an economy except as tribute, trading just increases your relationship with them.