Celebrim
Legend
Interestingly, at the beginning of some of the Welsh rebellions in the period after Edward's Welsh castles were built, they were often garrisoned by small numbers of men- 18-25 men-at-arms.
They made military-economic sense (you raised the economic issue on a later post) because despite the high creation cost, once in place they projected a large amount of presence for a fairly low on-going cost due to the small garrison. And in even this small number of men could hold off much larger forces long enough for reinforcements to arrive.
But in a D&D world, set your mid-level adventuring party the task of taking such a castle and how many would really fail?
This raises alot of points. How common of a threat is a mid-level adventuring party? If the threat isn't very common, then the castle might still be useful against common threats and hense justifiable. If the threat is very common, what is the chance that the castle is defended by a mid-level adventuring party? What disadvantage would a mid-level party be placed at, if they had to attack another mid-level party who were fortified in a castle and had at their command 18-25 men-at-arms of a level appropriate to the campaign world (both 'low level' and 'mid-level' are relative to the demographics of the population).
You could, of course, put a high level lord in the castle but that doesn't seem to be consistent with how those castles were garrisoned.
The question of how you'd have to garrison a castle given the common threats to it hasn't been raised that much (except when several people noted that gaurd dogs and other creatures with the 'scent' ability would be good investments).
We also haven't addressed the strategy of castle buildnig. Garrison castles as were used to project a presence into the Welch frontier, and deter incursions by the Welsh into England, and project English culture into Wales (by encouraging town development in their vicinity) aren't the only strategic use of castles. Even if garrison castles no longer make strategic sense (a point I'm not sure I agree with, since I think 'low to mid-level adventuring party' would be the standard garrison), that doesn't obselete the other uses of a castle like for example, defending a city or strategic point. A castle that housed the garrison of a cities military could be much more strongly defended than a frontier outpost.
Does the castle continue to make sense or are there other alternatives? At what point is the value in the defenders and not in the stone structure? When it tips to the former, why build the latter?
That is the key questions we are trying to answer. What are the alternatives to castle construction?