[tinkthank]

Howdy, and welcome to the dom2 Boards!

First of all, you may want to use the "search" button in the top right corner to search for "castle" or castling or similar terms, there were many threads on this topic which may be of more help to you than my humble words.

Now on to your questions, I hope I understood correctly:

Point a. Rarely will you find many (especially: many neighboring) provinces producing 100 gold AND 50 resources. This is a Rare Thing under normal richness settings.
Point b. You need also to calculate the 450 gold you are spending for your castle.
Point c. There are times when you really need a lot of resources, e.g. if you want to recruit, say, Ulmish or Abysian infantry, or especially in your capital if you want Marignon Knights of the Chalice. Units with 30 to 61 resources a piece will NEED neighboring resources or they will only be able to get 1-2 units a turn.

Otherwise, you are right -- if you do find a lot of provinces with really high gold production AND resources, you may want to build castles in them. However, it will be rare for a province to produce 100 gold without a mine, and it will usually nót have high resources too.

Don't forget, however, that you will need 450 gold per castle and someone to build it for 3 turns.

Did I understand your question correctly?