How to cheat a ck2 title

And my first question right away: In my small country I have two counties, which are currently divided into a castle and a town. I control both castles directly, the cities are ruled by vassals. The two castles bring in a relatively large amount of manpower and money, the building here would still be worthwhile for me, as the taxes or the troop strength go directly to me.
The cities, however, only pay me a fraction of their tax and troop strength to me. I cannot usurp control of a city, otherwise the city will be governed "wrongly" and no more money / troops will be transferred at all. In my opinion, the construction of buildings is not worthwhile in these cities (and probably also in the temple possessions), because even simple buildings like a harbor take many centuries to amortize (with the castles only 2-3 Decades).
Can someone explain the meaning of cities to me? I thought they should give you more income ...