To build a second city you need 110 health.
How do you get 110 health?
You can raise your health to 110 by advancing to the title of Knight
How do I advance to Knight?
You can advance to the rank of Knight by building your rep.
How much rep is needed for Knight?
How do I gain rep?
Refer to Common Questions>how do i get rep?
You will also need 5000 gold and 5000 food and the ability to transport it with your army.
So in total you will need the ability to carry 10000 resources.
Still having trouble?
Make sure your only trying to transport 5000 food and 5000 gold, no more no less.