Grietje Smit
What did the horses mean? Their colors, their glowy orbs for eyes? Etc.
Grietje: Has a white horse because her intentions were most noble and she showed the most concern for Anthy as a person rather than as a concept. The white horse represents purity and nobility, and also is shown repeatedly in the series to be the horse of a 'Prince'. Any of the other characters could have had a white horse if they'd shown the same convictions. The golden aura around the horse and its golden hoofprints were physical manifestations of the Power of Dios.
Rachel: Her horse is blood red because of her willingness to actually shed blood for Anthy, and it's the color I associate with sacrifice and determination. She also was given a measure of Dios's power as her desire to save Anthy showed itself more.
Adrian: His horse is ghostly because the horse itself is the vehicle of a savior, and Adrian's princeliness level was pretty low. It was therefore less corporeal than Grietje and Rachel's, and would have offered less protection against the attacking vines had Adrian faltered. Since he was limited in his complexes and assumptions about Anthy, he was unable to manifest a full 'spirit horse'. His horse was diluted in color, the original color would have been purple and red.
Maarten: He is lucky to even have gotten a horse. His intentions were not at all noble and entirely self-centered, focusing completely on the idea that he
should save Anthy and that it was his duty to, but with no actual desire to save her as an individual. His pure stubborn determination allowed the horse to form, which is why it sucked energy from him to do it: it was literally being generated from his will, and not from the magic set down in the arena. The color was dark grey because that's seen as a neutral color, neither black nor white, and the eyes glowed golden because what he
desired to be was a Prince.
Grietje Smit
Also, what was the halo on the horses?
The halo was the Power of Dios manifesting itself, and it was only seen on the horses given that power. Initially, it gave the rider minimal protection against the attacking vines: it would shield the rider from attack only if the rider did not flee or depend on that ability. Against the dragon riders, it allowed them to be safe from an attack from the actual dragon, and allowed protection against the blades of the opponent.
Grietje Smit
Why were the riders on dragons?
Well, firstly because dragons are kickass. There are metaphorical and fairytale-appropriate reasons: dragons symbolize fear and evil, and they are often the opponents for princes to slay...but I also happen to love dragons.
Grietje Smit
Why did you have vines attack them? As opposed to anything else? Sleeping Beauty or something?
It was definitely very shades of Sleeping Beauty, but it also went with the rose motif of the school: vines and thorns. The vines were another test against the duelists' resolve, to see if they would be driven off or defeated by the first hurdle. They were meant to pick off the doubtful.
Grietje Smit
And what answer was the disembodied voice looking for when it asked, "What will you give?"
Well, it's really hard to express in words. It was kind of to see what each duelist thought they could offer Anthy. Grietje basically said: "Anything she needs" which was kosher, I guess. XD