If PC's use the Stag Lord's fort as a base to start there first city, in Rivers Run Red it says that it halves the time to build a castle. From my understanding, if you build something in a city, its there at the end of the phase (and its assumed you've been building it beforehand, at least that's how I take it). So does the fort instead halve the COST of the a castle if you start the city there?
Same.
The location also grants a +1 to all three kingdom stats if your capital is there.
Personally, I gave the party a free watch tower - and told them they could spend money on it to upgrade it to a castle later.
But then I play a bit loose with the build rules as I let all sorts of building get upgraded :)
ITS A TRAP!
Yes its a 1/2 cost castle, its also the 2nd best place to start a second city. Don't believe me re-read the farmland rules.