When the Roman empire collapsed in the West, most of the cities were abandoned and empty. The population of Rome was reduced to about 20,000 people from about a million after the collapse of the empire. London was left largely empty too, since the Anglo Saxon invaders feared the cities as abodes of ghosts and demons. Paris could not shrink much because it never was the capital of the Roman province of Gaul. It was Clovis I who made Paris a capital.
You need urban centers to support guilds, shops and markets. Most people in the early Middle Ages grew their own food and there were few means of preserving anything. You can imagine that it would be like at the end of summer when you are crossing the road to share your excess zucchini and tomatoes with your neighbor and find them coming the other way with a basket of zucchini and tomatoes for you. You could try to sell them, but who would buy. Occasionally traveling merchants would pass through town and if you had some gold or coin and they had something you wanted, this would be your opportunity to buy, until towns started growing again. Kings began to give charters to towns to run their own affairs and to have fairs, that is annual markets. Since kings were rulers over their people not the land, these fairs were a source of tax revenue for the kings and is partly why the Crusaders needed Alexius to give them a market.
Of course the Eastern part of the empire did not fall with the West and so probably still had trade and shops but when 50,000 hungry men show up on your doorstep (or however many people actually appeared with the Crusader army) the local bakery is probably not going to be able to keep up with the demand for bread. This brings me to the topic of bread. At Nicea, the men, not having any bread to eat, sat quietly on the plain before the city. Once the bread showed up, the men, glutted with carbs, attacked. Frances Moor Lappe, who wrote Diet For a Small Planet, wrote that meat makes people aggressive. Clearly it is carbs that do so. Stay away from the bread!!
Aggression leads me to the third item that I would like to point out - the flinging of the heads of Turks, who were killed, over the walls. I do not know if Tolkien was acquainted with this work or if this was a common act in sieges, but the orcs did this when they attacked Minas Tirith in the Return of the King. 'Our guys' are the orcs.
I found a nice webpage with four accounts from the Middle Ages about markets. It is worth taking a look at. http://www.fordham.edu/halsall/source/1250medfairs.html