when should you build a new fortress?
 

As soon as you get enough money or wait (?) turns?

Re: when should you build a new fortress?
 

there are no general rules, but sooner is usually better than later. a second fortress lets you ramp up your research and your army recruitment - for some nations, like mictlan, it is especially vital. for others, like Vanheim under a bless, it is less so.

Re: when should you build a new fortress?
 

Short answer: It depends, but I'd probably build the fortress as soon as possible.

Longer answer: Why are you building the fortress? How many fortresses do you already have? Are you currently engaged in any activities (WAR) that could require the gold that you're planning to spend on the fortress?

What race are you playing? (Different races get different fortresses, and they also get different benefits from their fortresses.)

Re: when should you build a new fortress?
 

It also depends on whether or not your national units that can be built in a second castle are horribly important to you or not.

Other factors are...

1) taking a province that is in a chokepoint location

2) does a province you have taken have a unit important to you that needs more resources for best use

3) did you take a province with MANY connecting provinces

4) is there an ally behind you and needs a turnstyle castle in order to get troops out where they can be of use to both of you

Re: when should you build a new fortress?
 

Lot's of fortifications = lots of income. It takes a while for the fort to pay itself back, but they are the key to making that income curve climb up.

Re: when should you build a new fortress?
 

Depends on the kind of fort you are building. If your fort has an admin below 10 you really aren't going to get much extra cash even if you have a high pop place to build it. (forts add admin/2 % to income correct?)

Though generally the forts built in plains have decent admin values...

Re: when should you build a new fortress?
 

You should build a second fortress as soon as possible, so you can recruit another mage per turn if nothing else.

The 3rd/4th and even 5th should be built as quickly as possible too, for exactly the same reason.

After that, I personally just place my forts to defend strategic locations and very important gem sites only. I prefer to capture fortresses from my opponents from then on.

Re: when should you build a new fortress?
 

As was already said, the second fort should be built as soon as it doesn't hurt your early game. For fun, try coming up with a design and early game strategy that lets you start building a fort on turn 3 or 4 while still doing your best to expand and research.

Re: when should you build a new fortress?
 

3 or 4? will it not hurt the resources of your capitol to build it so near?

Re: when should you build a new fortress?
 

Hmm... you could start building a fort 2 provinces away from your capital on turn 5 - turn 4 if you went out blind on turn 1.

I guess beyond that the answer is 'yes - it will hurt resource generation in your capital'. But for races without high resource capital-only units (and ones you want/need to build, at that), it's not a big issue. The overall amount of resources you can put into troop production increases, after all.