These 15-year-olds have known each other since childhood but they are not friends: Elena is scathing about Beth's height, looks and prospects, vicious and demanding with servants -- including her Saxon maid, Alice -- and convinced that the baron intends her to marry Gerrat. Gerrat is not long in appearing (page 5) and it becomes obvious (except to Elena) that his fancy is fixed on Beth.
Next morning, after the three young women have all dreamt of a haunting episode amongst the nearby standing stones, Arbor Low (best to spell that Arbour Low for the English flavour), they learn that the celebrations are postponed because Lady Johanna de Ashbore, Gerrat's mother, has suffered a miscarriage. Nervous, but curious about Arbour Low, they go there, only to be enclosed in a large crystalline sphere.
Kim, you handle pace well: no reader could be disappointed by the way Beth's adventures take shape. Serious matters are already at stake on the domestic level – a young man has become a knight; his marriage pends; a sweet, inexperienced girl (Beth) is looking to assert herself as an adult; the young man's mother miscarries at a time when Beth's mother, too, is about to give birth. With some skill, you've created a workable basis from which to escalate the plot to matters such as legitimacy, political power, love, leadership and eventually civil war. And you've unleashed the supernatural element. From page 21, Beth's world seems about to be invaded (or engulfed?) by another, via the pulsating membrane inside the sphere.
PORTRAYING THE LOVERS
The 12th-century framework of your story is convincing and with careful maintenance will hold the reader's interest to the end. I have suggestions that may help strengthen it, if you plan another draft. Beth is a satisfying heroine and we're rooting for her. I suspect you want us to take longer to admire Gerrat. This is fine, but beware of undermining him at first. He's 20 and has just been made a knight after years of martial and courtly training, but he doesn't act like one. His knightly celebrations may include ritual jousting, so he's justified in begging a token from a lady before he fights – but demanding two, then hacking off Elena's plait, is an insult to both girls, and the antithesis of chivalry. Putting a viper in Elena's palm has sexual overtones that make his gestures obscene as well. Her family would have departed at once and Baron de Ashbore might even have had a blood feud on his hands. This episode is very easy to revise because you have set the dialogue up well: Gerrat simply needs to achieve his goal (two locks of hair) by being clever, witty and manly.
I also think it's important, when Beth first hears about the miscarriage, that she realises Gerrat has just lost a baby brother or sister, and that his mother's life may be in danger. Beth's in love with Gerrat and obsessed by him; any big event in his life is going to matter to her.
Alice has great potential as a character and, like Beth, is not hard to admire. The girls are Norman-bred while Alice is Saxon and a 'servant', therefore their inferior in language (they'd be speaking French, which she might have some trouble with) and standing. She is far too outspoken to be anyone's maid (especially to a termagant like Elena) so I'd suggest that instead she serves as Elena's companion, being the daughter of a merchant or former Saxon lord, perhaps, brought into the household by Elena's parents because of her intelligence and integrity -- they must know their daughter is a handful! Make sure the reader realises that Alice won't be dismissed for telling Elena the truth, and you can plausibly keep her in the story. Alice's sincerity (and perhaps genuine care for Elena underneath?) endear her to Beth, who allows her the familiarity you describe.
HEIGHTENING THE DRAMA
The moment we leave the domestic squabbles of Tissington, you plunge us into the life-or-death struggle at the henge. Again, try to avoid undermining your own effects. For a noblewoman like Beth, brought up under strict medieval religious codes, this whole experience would be terrifying beyond measure. Therefore, you might consider deleting this clause: 'she prayed and hoped that they had opened the gates to the underworld.' No one in her plight would be praying and hoping for this, in any era! Though she may have heard of the Greek notion of Hades, and the world of faerie, the only underworld Beth could really believe in would be Hell and its demons. Interestingly, medieval Hell was described as a place of darkness, conflict, fire and ice. Still more interestingly, it was also described as a series of descending circles. Trapped in those circles, mortals who had chosen the Evil Way in life were eternally punished. Hell was also pictured as a black vortex spun by a howling wind, and you cleverly use the vortex image in your piece. Monitor Beth's reactions here. I question how she could believe that the 'flashes' she sees in the 'swirling grey tunnel' show the 'yet-to-be'. Isn't this giving the plot away too soon? I suggest that at this stage in her venture/quest/love story, Beth would think she was looking not at the future but into the mouth of Hell, which would throw her into the utmost fear and despair.
The miraculous tensions in your story are very effective. At one moment Beth feels a flicker of warmth from the Gerrat-like figure on the other side of the divide – next second she's about to be lost in a demonic maelstrom. You're prompting the reader to wonder: What's going on?
I'm sure you'll draw your audience further into Netherrealm if you keep up these striking dramatic contrasts throughout. Good luck with your story, and I wish Beth and Gerrat well.