I actually assumed most of them were from the Reman Dynasty, at the end of the first era. Sancre Tor, being the tomb of Reman III, surely was.
It wouldn't make sense for them to have been built, or even used, in the third era IMO; But the second era for a lot of them could be possible. There was enough military strife in the second era to allow for it.
I don't know when they were built, but I'd like to know why they chose such odd locations for these fortalices. You'd think they would have strategic locations to protect important routes or operations, but they seem to be just here or there.
I justify that by thinking that "strategic locations" have changed over time. Maybe there was some previous relevance to an area that is now just forest. :shrug:
That would make more sense if most of these weren't built by a cohesive empire, but rather by warring feudal kings and whatnot. There was a lot of that in the 2E, if I recall.