Well, I built something like 60,000 troops in that city with all the barracks (the maximum I could do without building more barracks at the expense of local food stocks, etc), and then started building a castle. Man, it was like blood in the water in an ocean of sharks. My castle is getting attacked by like a half dozen different players, one of which already hit with over 200,000 troops in a single attack.
Wow.
That city/castle will be a complete loss. Live and learn.