It is possible to defend your city with zero chance of being conquered! (I am a war gamer for many years and while I do not program, I can see weaknesses in games and find ways to exploit them)
No matter how much is sent to attack someone, a city can easily defeat almost any amount if it is actively defended and reinforced.
I was part of the largest attack on anyone in server 85 a couple weeks ago. I was on the defensive side of the fight and I utterly destroyed over 6 million troops that tried to hit my main city! I lost very little from the attack. This is one of the games weaknesses in that defense is very much the answer to being a good player.
This is a kind of bug that is basic to the game play. It is not really a bug but is an easily used flaw in the game that makes a player who is online unable to be taken no matter what forces are used against it. Even if there are no troops or wall defenses protecting the city, the city will not fall.
The secret is to have a warehouse in the city and access to enough food to use disaster relief every 15 minutes. In this way, no city can be taken. It takes 30 minutes once loyalty hits 5 to reach 0 loyalty. In that time I can disaster relief twice which not only resets the timer for the loyalty reaching 0 but gets my loyalty back up to 10.
So without any troops or defense, just sitting in my city and using disaster relief I can avoid losing my city so long as I can stay awake, protect it and have access to enough food.
Some of the players have issues with the fact that you can not take a city while a player is active. It does not make sense that 1 million troops standing in a city fighting against no one, do not have the ability to take the city. But that is for the designers to decide what to do with. I am a player and will use whatever I can to make myself better able to play the game and keep my troops and cities alive.
Also even without my wall defenses, I can still get reinforced. I keep my gates shut and then once reinforcements arrive, I can open my gates at anytime I wish too. So I lie in wait for a wave I can destroy. Open my gates just before it arrives and kill it. Then as more powerful waves arrive I close the gates and wait for another wave to come that I can kill. Repeating this as needed to destroy enemy forces while not wasting mine.
Chuck Carroll Vran the Host of Niflheim Server 85
