[Omnirizon]

I actually like to hold off until i'm into the second year to start building a second fort. But I'll then build two in quick succession so that I have three before turn 20.

All through the first year you will be able to exhaust your gold only recruiting a mage and as many troops as you can each turn, and you should. This will allow you to expand a little quicker. While I'm still in expansion phase I am unconcerned with spending gold on two mages a turn (not to mention a fort to allow that), and would rather spend as much on troops as possible. The extra expansion will pay-off in research shortly when you actually have the extra gold to recruit two mages and enough troops.

By the second year you will usually start having a little left over gold, especially if you recruit efficient (i.e. the best survivability to gold ratio) troops. Your empire will also be bigger and you will enter a spatial situation where you can benefit from recruiting closer to your front lines. With these two conditions it is time to start planning to build not just a second but a third fort; both almost concurrently. An additional bonus of this is that you will be able to choose prime real estate for your forts, rather than just whatever is available. For me, building a fort in the right place trumps the cost or build time, and is worth waiting for.

Maybe this is just my playing style and the nations I like to play (I hate nations too dependent on their capital, and like ones where the placement of additional forts can be very meaningful), but I find it is what works for me.