The year is 1155 and my effort to unite all of Eastern Europe and Asia Minor under the rule of the Russian Orthodox Church is nearing completion. The only thing seperating me from my goal are the stubborn Turks who do not wish to give up their remaining few cities without a fight. In their last desperate attempt at salvation they arrange for a Holy Jihad to be called for my new capital - Konstantinopoli.
Soon enough a few Muslim armies show up but are utterly destroyed. In another few turns the Jihad is over and I forget all about it, until about 15 turns later a Moorish army shows up out of nowhere, complete with ballistas, heavy cavalry and what not. I rush all the defences I have at hand to protect the Capital, recruit all avalailable mercs and prepare to be besieged. The Moors stop right in front the city, but do not siege. So I wait. A turn...two turns...five...ten. Why aren't they attacking? The crusade IS over and we ARE neutral afterall (never even seen them before). At the same time the upkeep of the defensive army alone is costing me about 3k per turn. So eventually I forget about the Moorish army, just let it hang there. My defending army is either being called to different locations, disbanded or replaced by free militia units.
And then it happens. About 20 turns after arriving, the bastards attack my Capital. I have a general, and 5 militia (archersx2/cross/spear/cav). There is no time to bring in reinforcements so I have to fight. I burn down their siege tower and destroy the balista, but 3 ramming machines can not be stopped from destroying my gates...it's over.
They take Konstantinopoli and exterminate my largest city just like that.
The morale of the story: never trust a foreign army that enters your territory. Destroy them as soon as you have a chance.