Just to point out you need 10,000 of each resource INCLUDING gold and own a flat. You also need 250 workers in your Main city.
Click on the flat you've conquered & simply click on "Build City." If you have everything required, the build will begin.
CITY - RANK
1st city - Civilian
2nd city - Knight
3rd city - Baronet
4th city - Baron *Need the title MAJOR to become a Baron.
5th city - Viscount
6th city - Earl
7th city - Marquis
8th city - Duke
9th city - Furstin
10th city - Prinzessin