How would a city built in a ring fort or circular wall work?

The idea is that you have a wall that surrounds a city in a circle, with a gate on either side. The city itself is ring shaped, with the inhabitants living between the wall and the river. The wall wold need to be hundreds of feet high to provide adequate protection in a medieval setting, but I can't think of how this would work logistically. For example:<br><br>What would the wall be made of?<br><br>How would people enter the city? A single gate at either side would be impractical, so how many gates would there be?<br><br>How wide and deep would the river need to be for the river to be a successful barrier between the wall and enemies?<br><br>How would the economy work? Would the city be self-sufficient, or would there be a lot of trade?<br><br>Finally, what would the day to day life of a citizen be like? How would buildings be organised, how would agriculture work, etc