Yes that's right - she's actually a sidhe (born as one) but has no awareness of her own power or identity as of yet. Still, she has the hunting knife for now...

Let me look it up and I'll edit here with the answer - the population changed so drastically all the time it's difficult to remember!

Possibly; although the Marcher Lords were as much a pain in William Rufus' side as they were to the Welsh - Rhiannon will certainly be quite... xenophobic however.

I have it at about 30,000 - the 20,000 figure may well be referring to the post-siege population (historians generally accept over 10,000 were killed after the siege, although the estimates vary from 3,000 to over 70,000).