Hi all,
I'm not sure if this is the best place to ask this type of question, but this is something that I've been thinking for a while. Starting with ancient Rome and all the way up to early Medieval period, there have been a lot of examples of large fortifications being constructed - from walled cities, to purposely-built forts to extend control over territory.
How effective were these forts/etc considering the massive amounts of manpower and cost associated with their construction verses dynamic forces?
My main thought process is that dynamic forces/raids could ravage the country side while walled cities and forts would have little effect on that. That being said walled cities and forts would be a huge benefit against a large invading army.
I would imagine that it is difficult to say whether one is better than the other and the real answer is that a combination of both is probably the best of both worlds. Defence from sudden attacks and the time it takes to raise an army vs holding a standing army at all times come to mind...
How effective were these forts/etc considering the massive amounts of manpower and cost associated with their construction verses dynamic forces?
First of all let us establish something very important here. Almost all castles are more than just a military installation with a defensive purpose. Castles were centers of administrations, medieval military bases, symbols of power, toll stations and sometimes basically a city in a castle. Castles were in other words built for more than just defensive purposes.
That said, let's move on to your question about how cost effective they were as defensive structures versus a mobile force. The answer, I would argue, is not very. Some of the most famous examples of what you're asking about are the great chevauchées of the Black Prince in 1355 and 1356. He led a force of several thousand men on what was basically a great raid across the french countryside. Rather than engage him directly the French fortified themselves in castles and walled towns. The English simply bypassed these locations and picked the countryside clean instead. On the second chevauchée the french did pursue, this resulted in the battle of Poitiers which didn't go great for the French but was probably still preferable to inaction.
Still, the very reason chevauchées were commonly used is simply because castles are not great protection against them. A chevauchée can bypass a castle and devestate the countryside because it is fast moving and there for a limited time, an invading army can not. An invading army can not leave substantial forces in well defended positions in their rear without leaving its supply line exposed.
So no, castles were not a very cost effective counter to mobile raiding forces but they were never really meant to be. They were still worth building for other reasons though.