Always try to build gates instead of walls around your city.They cost the same but
the difference is between their attack chances.If a wall is attacked 1 time, it is
attacked 0.001 times.They will just come and roam near your city outside but cannot
attack your city.Always try to make your city wide open because they cannot attack 
the gates but will ataack the incities buildings like houses,barracks and all 
buildings by means of archers.mangonels and trebuchets and also do not make towers
and castles for defense near your gates because the gates will not be attacked 
and you can progress without defense until you can attack.So your first priority 
should be surrounding your city with gates. and capture huge areas and resources.
BEST LUCK!!!!!