Thick hooves beat the hard ground as Atrius swept towards the approaching darkness. Alys, who had never before known the true absence of light, was surprised to find that her eyes were adjusting to the dimness of the penumbra. Whilst what surrounded her was a colourless world, she began to become aware of a variety of greys within the darkness, and she soon found that she could see quite well in the dimness, which she found slightly disconcerting, but not unpleasant. For in the gloom was a world she'd never seen before, and Alys was possessed of a natural curiosity and often took delight in new experiences that her kin disapproved of.
The world Alys grew up in, a world bathed in perpetual sunlight, was hot and arid. Vegetation was sparse and typically zerophilic, and the animals she had encountered thus far in her life were grazers adapted to living in such a place. Horses, dogs and oxen were very common during her childhood, yet very few other animals were, so for the first time she was seeing a variety of creatures she found both strange and wonderful. Due to its cooler climes, vegetation grew much more readily in the penumbra, and so a larger variety of animals were drawn to feed, animals Alys may have eaten or worn during her life, but had never seen living in their natural habitat. Smaller, woolly animals chewed contemplatively on grass, these Alys recognized as sheep- they occasionally looked up as her horse rocketed by them, but without alarm or concern, returning to their leafy meal as she passed them by. Cows she recognized also, and wild horses too, but she also saw creatures she had no prior experience with, but again never felt threatened nor disturbed by them. Great gray creatures with woolly hides and long noses, small dog-like animals that sniffed in the undergrowth, all these and more she caught glimpses of as she continued her journey toward the Frontier.
Yet it was the birds who fascinated her the most. Alys had previously been afraid of birds, as most or her people associated them with carrion scavengers, vultures and condors in particular a continual factor in the life of a desert-dwelling nomad. But here! Birds were tiny, delicate creatures, flitting from tree to tree, chirping in a sing-songy voice that was a thousand times removed from the harbinging cry of warning from the birds of prey she had come to know and fear. She wanted very much to bring Atrius to one side and spend some time looking at the birds, perhaps singing back to them to see if they would respond, but she sensed very strongly that she was close to the Frontier (for surely it could not get any darker, she reasoned, in spite of the fact that she could see the mighty swell of true night rising up before her on the horizon), and she wanted to press on and get to her intended destination before having to sleep in such an unfamiliar landscape.
If she tried very hard, Alys thought she could vaguely recognize places she had been, skylines she had seen under a blazing hot sun. They were more impressions than actual memories, an underlying sense that she had trodden these hills before. And in all likelihood, she had, or had at least passed close to this place before, possibly many times. She was still following the line of the great circle, after all. But that would have been many moons ago, before the vegetation had grown, before the animals and the shadows had come, so in a sense it was a very different place to her than it was when she and her family had passed through. She wondered briefly if her birthstone was now in darkness, and what strange manner of demons must be swarming around it now if it was. She forced her mind to move on to other subjects- thinking of what dwelt in the darkness was not a habit her people much enjoyed.
After much swift travel, and much nervous internal debate about whether she should stop for a short rest with Atrius, Alys saw a strange light glowing ahead on the horizon, framing itself against the background of darkness as a flickering point, later coalescing into a solid flare floating strangely, some distance above the ground. At first this made her quite frightened, for even though she had been told of the powers of the Enemy, her imagination still conjured abilities and machines of which she had not been told, and her imagination, seeded by the night all about her, ran wild with the thought that the light was the gleaming eye of some enormous creature she dared not even contemplate. When I say that it took her quite a few minutes to recognize this phenomenon as the light of a fire, please do understand that she had never seen fire in the darkness before, and if you have seen fire in the light you will know that it actually looks quite different when it has nothing else to compete with for illumination.
Once she recognized the flame for what it was, her pulse slowed dramatically and she urged Atrius on with more vigour, realizing that she had, at last, reached the Frontier.
Ah, the Frontier! While grotesque in its way, it was also a wonder of the world in its time, and I find it difficult to describe to one who has never gazed on it.
As Atrius gathered his strength into a final burst of speed, Alys began to make out the gray semblance of a tower below the flame- the light was a signal flare, not only to illuminate the space around the tower, but also to signal to other towers in the Frontier. Alys turned her head to the left and to the right, and sure enough, as the edge of her vision to the North and to the South, she could see the tiny flames of two other towers, one in each direction. It would be the same for each tower that formed a tiny part of the Frontier, each would see two brothers, one to the left and another to the right, and it would be this way for many, many miles, for the Frontier consisted of over a hundred towers, each connected to the others by a long chain of vision. By manipulating the signal fires atop each towers, messages could be passed up and down the tower line by a relay system, so that every company on the line would be aware of Enemy movement or attack at any point along the penumbra, and could respond accordingly, sending troops of men up and down the line to the tower where they were needed most. The Frontier was her people's only line of defense against the great darkness -and while they could never hope to defeat an opponent so numberless, they were certainly an imposing, and forbidding, series of structures- they gave Alys' people the freedom to move about on the light side of the umbra, to gather grasses and fruits for food, to tame wild horses and bring them back into the light, to slaughter sheep and cure the meat, to chop wood for transport to Cardinal ciy and the land trains, to gather milk from cows to make cheese and cream. Alys' entire world depended on the bounty of the penumbra, and that bounty was kept secure from the Enemy by the Frontier. Alys had heard of the glory of the Frontier all her life, and the sacrifice of those who chose to serve at the borders of her world, so to see it in person first hand was a moving experience for her, like meeting a hero you have idolized your entire life.
As she got closer to the towers, she heard a deep rumbling coming from the base of the tower ahead, and began to make out the rank upon rank of oxen that was deployed at the foot of the tower, each drafted into a mighty yoke, which was ultimately attached, through a sophisticated series of ties, to the tower itself. As each ox strained against the yoke, the tower rolled on enormous wheels, westward towards the sun. The towers themselves moved very slowly, about the speed of a walking man, but it was fast enough to keep pace with the sun, to keep pace with the umbra, the dividing line between the darkness and the light. About the tower itself, unbridled oxen grazed lazily on the grass, or slept. These would be rounded up by bullwhackers when the oxen currently pulling the Frontier away from the darkness began to tire- each ox was kept on a strict rotation, although after a lifetime of hauling a huge construction like the tower, they were hardy beasts indeed.
Alys was now less than half a mile from the tower, and was beginning to enter the field of light its topflame was emitting. No sooner than she did so, a squad of men on horseback, about half a dozen of them, silently emerged from the darkness to the right of her. Her encounter with Borin still fresh in her mind, Alys' heart leapt into the throat as the cavalry made a beeline for her position, coming up alongside Atrius as he galloped towards the tower. They wore the uniform of Frontiersmen, the most hardy and respected soldiers in the King's army, but then again, so had Borin.
"Ho there! Haw up now."
Frightened, but seeing little alternative, Alys applied a series of checks to her reins to bring Atrius down to a mild canter, whispering quiet reassurances to him as she did so, then pulled him up to the right to turn to a stop and face the soldiers, who quickly formed up in a rough semicircle around her. They kept their spears across their backs, seeing no need to draw their weapons for someone who was so clearly not the enemy. The horses before her wore the livery of the Kingdom, the shining yellow sun of Helios visible faintly by the light of the watchtower. One of them was more brightly uniformed, with a series of spears clashing across the image of the sun. It's owner pulled off his helmet as his horse stepped forward from the six- it was the same man who had called for her to stop.
"You are at the Frontier, little one. None but demons and criminals dare pass this point. Be on King's business or be gone."
Alys stumbled nervously over her words.
"My name is Alys. I'm a medic, I've been assigned to my brother's company."
The man's eyes darted up and down, reappraising her in light of this new information. His men relaxed and started to fan out from her, looking about them with the eyes of those trained to scout for danger.
"Well, we're certainly in no shortage of need for more medics, welcome. I am captain Ganroc. What company does your brother hail from?"
Alys began to breathe more easily, certain that this Ganroc was not a threat to her.
"He is part of Kelron's company, have I found the right tower?"
The captain laughed and shook his head.
"Not quite, little one. Kelron's company patrols with the King's forty-ninth tower -you have found the fifty-third." He pointed his helmet, still in his hands, towards the tower on the horizon, it's topflame a tiny candle in the distance. "The one you want is four towers down the line, that way."
Alys' heart sank again, a look Ganroc seemed to apperceive and take pity on.
"You look like you've had a long run, you came from Tarkos?"
"Yes," Alys looked down "many miles sunward, but I began my journey at Cardinal City."
"Cardinal City!" Ganroc declared, obviously impressed. "You must have been traveling for a few fair moons now. One hell of a long way. I've not been to The Great City for many passes, many passes." he drifted off momentarily, clearly lost in memory before quickly returning to the present. "You were born there?"
"Yes." Alys said, supplying Ganroc with an invented family name. It would serve her no purpose to have soldiers on the Frontier know who her father was- she did not want that part of her old life following her into the new one.
"Well, you clearly look like you could use some rest. Come, I shall escort you to the watchtower personally. We'll stable your horse, he's a beauty but looks like he could use a rest, and if I dare say so you look like you could use one yourself, and possibly some hot food, yes?"
Alys nodded gratefully, the mention of food setting her stomach off, a reminder that she had been living on dry foods for quite some time.
Ganroc barked a few orders at one of his men, telling them to continue circling the tower, and to be on the lookout for the Enemy. His men each reported their assent with the military cry of "Ho!" before setting off again into the deep shadows.
Ganroc's horse sidled up beside Alys', which Ganroc now addressed directly:
"Oh, you're a pretty one, aren't you? What's your name?" he said, leaning forward in his saddle to pat his nose.
"Atrius," Alys answered, since Atrius was unlikely to answer for himself any time soon.
"Well, you'll get a good chance to graze while your mistress sleeps, my friend. If there's one good thing to say about this dank doorway to hell, it's that there's a lot of things you like to eat. I might even be able to get you a salt block, would you like the sound of that?"
Alys could still feel the Atrius' tension beneath her, both from the unfamiliar terrain and his encounter with the wounded man, but even so he snorted his agreement to Ganroc's suggestion.
"And let's get your mistress off before she falls off." he continued as Alys struggled to keep her eyes open.
Together, they began to gallop towards the ever-rolling tower.

Hmmm... The world takes shape.
I love the idea of a line of towers being pulled continuously away from the ever-encroaching darkness, even though when I use the logical part of my brain it's clearly impossible. But then that's what fantasy is for, isn't it?
That depends what you mean by impossible. If you're saying that it's impossible to to outrun darkness by moving away from it at walking speed, that would depend on the speed of the orbit of the planet you're on. Most heavenly bodies turn once every time they orbit whatever it is they're orbiting (usually a star), which means that not only can you easily outpace the umbra (the dividing line between night and day), it stays almost perfectly still. So it's not hard to conceive of a body that has a very slow day, slow enough to walk and keep up with (indeed, the Earth will one day be in this state, since each day is imperceptibly longer than the previous one- we are slowing down).
If you're referring to the impossibility of oxen dragging a wheeled tower, I specifically didn't mention how many oxen were dragging so that, regardless of what math you might apply, there could always be more that you'd need. I'm imaging about 20-60. Indeed, Siege Towers were often pushed up to castles with as little as 20-30 men pushing them forward*. Admittedly what I am imagining is somewhat larger than the flimsy wooden towers used in sieges, but the principle is the same.
The only other thing I can think of is that you couldn't actually have a continually moving group of oxen, as they'd fall behind the line as they slept/grazed. I just assume they're 'double-timed' up to the towers when their shift starts, and that, due to their role in pulling towers all day, they are a hardier bunch of beasts than the lazy bovines we're used to. I mean, I could have called them Corcophacs or some other invented name to give them assumed unnatural properties, but just take it on faith that, for this world, they are the equivalent of what we call Oxen.
Also worth consideration is that the towers may occasionally stop for rotating the oxen, picking up food and so forth, so long as they then resume their movement at sufficient speed to outpace the darkness. As a typical rule of thumb for the people of this world, once the sun dips below the horizon, you'd best start heading west, and fast.
*your average ox pulls at about 2hp, or 1100 foot-pounds/second. Get 60 of `em going and you've got about 66000fps, or Quite A Bit. Not much torque compared to today's car, but enough to drag a jumbo jet around the tarmac a walking speed, no problem.
No, impossible as in unless the planet were perfectly (or at least relatively) smooth over it's entire surface, with no oceans, lakes or deep rivers, mountains, valleys or snow-covered glaciers, it would be impossible for the oxen to pull the towers continuously ahead of the umbra.
I'm sure it's theoretically possible, and in a fantastic setting it would be totally possible, nay plausible, but using logic and reason, it's completely impractical to the point of impossibility, no matter how many oxen you had.
Yes, the Frontier does follow a rudimentary path that is navigatible by ox-drawn towers, among other things. While the nature and creation of this path will be made more explicit later in the novel, it's not inconceivable that a natual land bridge of relatively flat land might form on a planet that turned very slowly, as the extremely hot days and freezing cold nights (a series of deserts and frozen wastelands) might conceivably 'scour' the surface, making them much flatter than our comparatively vibrant world. Many of Saturn's moons, for example, are almost eerily flat, and you could walk about them no trouble. That is not to say that there are not mountains and rivers on this world, just that the Frontier takes a path that avoids the unpassable ones.
Here's a rudimentary map I've made up to show the path- it is in a beta stage and should not be taken seriously until I've had time to think it over seriously.
Please explain why a line of towers moving at walking pace can outrun the darkness, but a person without a horse would be fucked.
Because a person walking would only ever be able to 'keep up' with the darkness by walking constantly, which a person cannot do, so they would inevitably fall behind as a person cannot continually travel at walking speed. Without a horse you'd be unable to travel conventionally in the lightside society- part of the bonus of the Frontier would be a place to rest whilst still moving. Part of the essential nature of having a horse or caravan (like all nomads) is that you would be able to stop for a time and then, by traveling much faster than a walking man, catch up with or even overtake where you might have been on foot. So if you don't have a horse, or something equivalent to do your walking for you, you're probably a bit fucked.
Also worth noting is that the Frontier doesn't outpace the darkness- it keeps in step with it. This is by design- if the Frontier were to start outpacing the umbra (which they're probably capable of, if they start whipping the oxen real hard, like), they would slow down a little.
Well! I see you've done your research, anyway. :-)
I'm totally looking forward to part three.
Did you know that another meaning for 'umbra' is 'Uninvited guest brought by an invited guest'?
No point, just a bit of trivia for you.
Interestingly, that alternate definition foreshadows the second half of the novel! It also means 'a phantom or shadowy apparition', which ties in nicely as well. Like, deep.