1. You must have gold...at least 200....(but this is probably not the number that you will have to spend, because you must have this ammount just to be able to build mark tower).
2. Open valley map...click anywhere in free location where you want to move your city and choose from up menu - build mark tower (this is free action).
3. When mark tower is building, you can click on the main building in the mark tower and choose what to do next - there you can also see all the prices.
4. Choose which city to move and after mark tower is ready, it will move your selected city.
(Note that you have to send at least one soldier to your mark tower before it is ready)