: Appendix A says that the Heirs of Valandil abandoned the city. THE PEOPLES OF MIDDLE-EARTH says that Amlaith of Fornost was the king who actually moved the capital to Fornost Erain (hence, the "Of Fornost" part of his name, I suppose). I'm inclined to accept that as "canon", even though the remark didn't survive into the final text of "The Tale of Years" in Appendix B.

: : My guess would be that Annuminas was abandoned over a period of
: : years, finally becoming deserted sometime after TA 1409. The
: : simple reason being that the Kings of Arthedain did not possess
: : the military strength to defend the ancient capitol of Arnor
: : (much as the Dunedain of Gondor eventually lost Minas Ithil and
: : Osgiliath to the enemy).

: I think we recently had a discussion where someone proposed that the division of Arnor may have resulted in many of Annuminas' people leaving the city. Maybe I extrapolated that from something someone else said. I'm not sure at this point. But it makes sense that if there were divisions among the Dunedain, the city may indeed have fallen into ruin because its people simply left to follow their various princes.

: : With the defeats of Arthedain by the Witch-king, the remaining
: : Dunedain were forced to seek refuge where they could - Fornost
: : Erain (Norbury or 'north burg'), Amon Sul and the forts along
: : the North Downs being the places which could most easily be
: : defended.

: Annuminas would have been pretty safe, however, being far removed from most of the fighting. It may be that the people simply followed the armies. Or perhaps Fornost was a fortress until Amlaith settled there, and then he just drew in as many people as he could.

: : It is suggested, however, that Aragorn planned to occupy
: : Annuminas in the FO and make it once again the capitol of a
: : restored Arnor.

: Well, he did actually go to Annuminas early in the Fourth Age, when he visited the Shire, and Elanor Gamgee stayed in Annuminas for a year, I think. In fact, in the second epilogue, Sam says Aragorn had visited the Shire when Elanor was "a mite" (presumably before she was 2-3 years old). Christopher says in an end note that he doesn't know of any other reference to Aragorn's earlier trip, although I think Gandalf's words to Barliman in "Homeward Bound" may be a hint of that trip:

:

:   'I suspect it has,' said Gandalf.  'Nearly all lands have been
:   disturbed these days, very disturbed.  But cheer up, Barliman!
:   You have been on the edge of very great troubles, and I am only
:   glad to hear that you have not been deeper in.  But better
:   times are coming.  Maybe, better than any you remember.  The
:   Rangers have returned.  We came back with them.  And there is
:   a king again, Barliman.  He will soon be turning his mind
:   this way.
: 
: 
:   Then the Greenway will be opened again, and his messengers will
:   come north, and there will be comings and goings, and the evil
:   things will be driven out of the waste-lands.  Indeed the waste
:   in time will be waste no longer, and there will be people and
:   fields where once there was wilderness.'
: 

On a similar note, could any other Numenorean fortresses and/or castles have been rebuilt/reclaimed?