How did Asriel set up his army so quickly?

[Somewhat]

Lord Asriel is the image of the classic Romantic antihero (http://en.wikipedia.org/wiki/Anti-hero). He therefore resembles Satan as the rebel in Paradise Lost, as he is in the verse at the beginning of the book. Mary Malone, on the other hand, resembles Satan as the seducer, as he appears in the Bible.

[Tranquil]

The infrastructure and everything might have been in a somewhat ready state, since Dust and Rebel Angels were planning this for eons.
It possible that just as Lyra had a destiny to fulfill, Lord Asriel too had this part to play and the Angels knew about that.
They knew that Asriel , with his passion against the Church, his leadership would be an ideal candidate for leader; just as the Angels/Dust knew that Mary Malone would be perfect to "play the serpent"

Of course, some things do give the impression that he built the fortress from scratch in that short a time and that would be impossible unless he had immediately assumed leadership of a massive workforce.

Either way, you're looking at some preparation done before hand.

~Tranquil

[moyeongsu]

Well, I think he's just a badass and he found the world without conscious life and just summoned them altogether. That was my image. Or maybe he really can make time slow down or speed up as he wishes, just like Ruta Skadi said it appeared to be. Mrs. Coulter snapped her fingers and the spectres had the ability to fly.

Asriel = Father Time, the ability to control time
Coulter = Mother Earth, control over living things

And they had Eve. Just throwing it out there guys

[Annernanner]

But are spectres really living.... they don't seem to be so much creatures, as hauntings.

Coulter really only had good control over men, but seeing as how men were in power at that time period, she had a quite a lot of control. Maybe she was some sort of metaphor for Mother Earth, but I think she was just a love-making, power hungry lady.