This is a fascinating question because it allows us to approach a historical period, the so-called Dark Age, between the collapse of the Roman Empire in the West and the establishment of the Holy Roman Empire through a specific lens--the building of castles. (I would, however, want to know why you chose the date 833. Is there a particular castle you have in mind whose construction commenced then?)
The best way to approach history is to think locally, or at least to circumscribe the scope of your question. Use this question to help you understand how to approach history. People tend to think in sweeping generalizations about history, but the best way to answer a question like this is to limit to a particular geographical area and/or political dynasty.
Identify some historical and geographical regions where there are many castles in the period after your proposed 500-833. Castile in Spain ("Castilla" in Spanish), for example, is an area with such a proliferation of castles that the region is named for it: the word for "castle" in Spanish is "castillo." On the Iberian peninsula, Visigothic rule filled in the vacuum of power from the breakdown of the western Roman Empire. Under a system of effective centralized rule, there is no need to have castles. Why? Think about what a castle is--at least the kind of castle that most people think of today, which is a structure from the High Middle Ages. It includes a walled fortress, sometimes surrounded by extra precautions against invasion, such as a moat. Inside, there is housing for the royal household and military retainers, as well as ample space to stock large supplies of food and drink and other provisions. There may be room for limited farming and housing for skilled workers and other laborers to survive a long period of besiegement. It's a place for the ruling and warring class to retreat in the likely event of an attack of an outside military force. Castles proliferate during the period of the collapse of powerful kings, like the Visigothic kings and the Muslim kings from North Africa, who were able to maintain order over large territories. As powerful as a castle looks, it really symbolizes very small power over a limited area.
In Spain, then, it is only after Islamic forces from North Africa invade the peninsula in 711, that one begins to see something like castles. And yet we don't have any castles that I know of from the eighth century. My guess is that is because the Umayyad conquest of Spain was so complete and covered almost all of the Iberian peninsula by the eighth century. When Christian Spaniards began to reclaim the peninsula piece by piece over a period of about seven hundred years, both Muslims and Christians erected castles in the border areas of kingdoms. Castile, for a long time, was a border between Christian and Muslim kingdoms and other kinds of domains (duchies, counties, etc.). This was a later time period, and that is why we find castles mainly, as in the rest of Europe, going only as far back as the eleventh century or so. Another possible answer to this question might have been that castles in Spain were made of wood during this time, and wood structures don't tend to last that long. But almost everything is built with stone or brick in Spain, as it is a rich place to quarry, but very impoverished in the natural resource of lumber. So, the answer has most likely to do with the stability of the Visigothic kingdom and then the completeness of the Umayyad conquest.
Although people tend to ask very general questions in history, the answers are often specific. Take England as an example to the same question. There were indeed castles in England during this period. However, unlike churches, which were built of stone, fortified castles were built of wood during this time. Anglo Saxons had a tradition of building in wood, since their situation was the opposite of Spain in terms of natural resources: there was an abundance of lumber, but not of stone. In this case, the answer looks very different than in Spain.
The key to answering your question is to limit the scope of your question to a particular time and place. In terms of England and the Iberian peninsula, the reasons for there not being a proliferation of castles are different: in the case of Spain, it has to do with the special political history of the area and with England, it has a lot to do with natural resources.
Finally, I did find an example in France of a castle that goes back to the beginning of the ninth century, within your time period: the Mayenne castle. Can you use the methodology of my responses--looking at a limited area, circumscribed politics, and the natural resources of the area--to answer why this castle was built during this time period?
