Risen Kingdoms is a turn based 4x styled game with strategic rpg battles and a persistent hero system.
Cities can be constructed, sieged, captured and destroyed making them very dynamic elements on the map. As you can see some structures that can be built in this city require resources unique to the human empire, this means if this city is captured by another race you wont be able to construct building that require one of these resources.
This means its often a better idea to destroy the city for resources or convert it to your race. Note: You are still able to build and recruit from another races city as long as the unit, structure, or action doesn't require that races unique resources.