It seems there are a large number of castles along the Rhine river in Germany. In some cases there are multiple castles in a 5-10 kilometer span. Why so many? How did all these people afford them? Where they built with forced labor/slavery? Was the chance of attack from barbarian hordes that high so that everyone that had enough money felt they needed a castle?
Hi - I just got back from this area of Germany (specifically the Rhine - Mouselle area). I wondered the same thing. I can only speak to this specific area of the Rhine so my info may be limited.
I know that this isn't a full answer, and as such, will probably be deleted. What I found out is that there are some sources of info on these castles, but it's very difficult to find especially in english. Much of the information that I learned was firsthand at these sites (I wont put it here as I cannot vouch for it's authenticity) but I will include what I did learn from reliable sources.
Just to set the general context: I'll be talking about the 11th-15th centuries and the area of the Rhine between Koblenz and Mainz as well as the area of the Mouselle between Koblenz and Trier.
Everything that I've read says that these castles were built to ensure taxation on the heavily traveled Rhine river. Nobles would setup a castle so that they could garrison it and collect taxes from merchants and anyone who traveled up or down the river. This is understandable when you realize that almost all these structures are built with a commanding view of their stretch of the river. Also, there are examples of watchtowers or boom chain towers built right along the river. There is even a mini-castle built on an island in the river.
Each Castle's history is different, but a good number of them were built by one family, the Katzenelnbogen:
Another thing to consider is that because this area had so much economic traffic there were always conflicting interests trying to win overall dominance in the area. Specifically the structure of the Holy Roman Empire at the time gave political power to the clergy in the form of the electors of the Emperor. It was just as common for a castle to be held by a Bishop and his troops as it was for one to be held by a local Baron.
Along those lines, depending on the political landscape at the time local rulers could have very little, or no oversight from a greater ruling body, and thus were free to make "war" on their neighbors. One example of this is the "Eltz Feud". I'm going to commit a horrible sin here and just link the Wiki page because all the sources there are in German so I can't read them directly.
Furthermore, many "Robber Baron" or "Raubritter" would setup these castles to tax, rob, and kidnap merchants.
I'll just do a quick summary to wrap this up:
Mostly they were built to enforce trade and passage taxes on people traveling on the Rhine
There was also a lot of conflict locally to protect from
Lack of a centralized government at times contributed to the proliferation of these castles
State and Nobility in Early Modern Germany: The Knightly Feud in Franconia, 1440-1567
The fragmentation of the former "stem duchies" Franken and Schwaben (the most prominent territories along the Rhine) lead to the minor and higher lords starting to build castles at the same time to protect themselves from one another, but also to expand the territory which they could dominate. For example, the Prince-Bishoprics Mainz, Trier and Köln were the most profilic builders on the Middle Rhine, thus trying to dominate the minor nobles there.
The stem-duchy Franken dissolved after Emperor Otto (the Great, I.) won against the duke, Eberhard (939) who rebelled. After this, Otto stopped to enfeoff people with it, practically abolishing the title. This lead to the smaller territories in the stem-duchy to see themselves as independent, with only the Emperor above them [this would later become the norm in some parts of Germany, being called "reichsunmittelbar"].
Over time, some territories were elevated to higher ranks, the most important in former Franken would be the Prince-Bishopric of Mainz (later Elector) and the County-Palatine of the Rhine (later also Elector), while no one getting the former privileges of precedence and dominance over the smaller territories. [To illustrate that, the Prince-Bishop of Mainz had no legal competences to prohibit to his neighbor, even if that was only the Reichsritter of Hinterletzteseck [only an example name] to build a castle. Theoretically, only the Emperor could do that.]
There were provisions against "illegal" castles in some of the Landfrieden - edicts, but they mostly only concerned castles which were "bastions of peace-breakers" (meaning Robber-Barons), but forcing someone to dismantle his castle would have been illegal. Such an action could lead to a Fehde, which mostly was bad news for all the involved. The friends and relatives (anyone really) had the right to help their friend in a Fehde, so such actions had the tendency to cascade into a small civil war. The minor nobles held together and often forced major nobles to give concessions. Countless castles were destroyed in such minor wars, or simply in disputes over inheritances or borders.
When the power of the Emperors faded (from 1050 onward) and finally broke down in the civil wars and the Interregnum (1254 - 1273), no Emperor had the power to prevent people from building castles. And indeed, most of the historic castles were built during that periode.
In Schwaben, the development was nearly the same, except that it splittered in 1268, when the last Staufer, Conradin died.
And after the Golden Bull of 1356 the decentralization was cemented, so no Emperor could hope to force the people to demolish castles.
Btw, nearly all of the castles on the Rhine were destroyed at one point or another, and all of the ones you can see now were rebuilt in Romantic style or even are newly build ones from the 19th century.
An overview is given in
Thon, Alexander "... wie ein Monarch mitten in seinem Hofstaate thront". Burgen am unteren Mittelrhein
As a follow up question, is it a similar to the Loire castles in France ?
Followup question:
Were heavily castled areas ever developed with the knowledge that an area is just good for castles naturally? Like, little chance of earthquake or erosion, etc? The Rhine Valley seems like such a place, and I wonder if it would have been as popular given the potential economic and political factors had the area itself been more prone to natural disasters.