I regret that I have not been able to finish Merlin's Nightmare in time for this blog tour, having read only half of it. However, I do wish to make some general remarks.
Without question, the tale is very good. Lots of things going on without any of it feeling out of place or extraneous. So far, the shift of focus is from Merlin to Arthur (Artorius), which had to come sooner or later. It should lead to Arthurian sequels with Merlin still in the picture but likely taking a less important or less prominent role (which is sad - as I found the transition from Bilbo to Frodo).
As a novel for young fantasy readers, the writing is probably appropriate. I have enjoyed everything I've read thus far. The battle in the south and the killing of Horsa by Arthur would wrest the imagination of most young readers, though I did find Arthur's removal of his boots in the heat of battle with a chariot and foot soldiers almost on top of him a little far-fetched (unless they were slip-ons, you just wouldn't have time to do such a thing). The story is a bit gory at times, but not so graphic that it is offensive - at least for me.
Morgana and the Voice come across in a very menacing, dark way; the transformation of Ganeida (Merlin's half sister) into Morgana has been developed quite well through the series. The despicable Vortigern, at least in the first half of the book, seems to be more down to earth. Not that he has become a likable fellow by any means, but you see a side of him that is less monstrous than what we've seen in the previous volumes. At the same time, Vortigern's capricious attitude toward the Saxenow is setting up for what might become a climactic conflict between him and Arthur.
The Picti of the north are an ominous threat, and Merlin, though he has been drawn away to the south with Arthur, receives puzzling signs of something amiss through the remnant of his wife's (Natlenya) skirt which he has taken as a keepsake. For me, his reaction to these signs (wetness, renting) are quite subdued and therefore unrealistic.
Again, I recommend the series for young fantasy readers.
In conjunction with the CSFF Blog Tour, I received a free copy of this book from the publisher.