Castle's 2012 London Olympic dreams evaporated when she sold her last horse, Gym Star One, unable to resist the urge to decline a lucrative offer from an Aussie buyer.
"I had a 12-year-old girl. If I had gone [to London] I would have lost my job," said the mother of Grace, now 16, and son Jordan, 18, a pedigree track cyclist who has ambitions of making the national team to the 2020 Tokyo Olympics.
"In my family dynamics it wasn't the right time ... to take a horse, you can't take it like a bike," she said, adding husband Ross was arriving today to support her, too.
"You go six months before. You can't just arrive there and unpack it [like a bike]," she explained. "I could have gone to London but my kids were 12 and 14."
Today she'll have another shot at a 70-plus score with Karlos (Magnus' name at home) during the Dunstan Horsefeeds CDI FEI Grand Prix Special from midday.
"It'll be a higher degree of difficulty tomorrow," she explained, adding she had ridden Karlos at that level only once before at GP level (yesterday).
Now she has the money to go to the Sydney CDI next month to accrue more qualifying percentages but she's comfortable in the knowledge she has a year to achieve that.
Castle has got her hands on at least a share of the $9000 up for grabs, depending on whether any other rider scores a 70-plus score to stake a claim on a share of that pie.
The prize money comes courtesy of Beth Bielski's 70 per cent club, which was introduced three years ago as an incentive for Kiwi riders.
She is quick to juxtapose that with having to make 90 per cent in Europe to make the world stage.
"You'll definitely make the top 20 and make the final, which will be most exciting for a Kiwi but the whole journey to Rio is massive," Castle said, in dialogue with the 17-owner syndicate to make it happen.
Way, she understood, was gunning for the World Championship in Canada in 2018.
"The pressure off me is just immense. I love it.
"I can't believe how much the pressure levels dropped in the last half hour.
"It'll be about getting back in the zone tomorrow morning."
Her favourite movements at GP level are Piaffe - a calm, composed, elevated trot in place - and Passage - a very collected trot, in which the horse has great elevation of stride and seems to pause between putting down its feet (like "trotting under water").
A performance coach for New Zealand Eventing, she attributed her success to training time with world and European champion Charlotte Dujardin earlier this year.
"She has brought another whole level of training to us, and lifted our team culture to top level," says Castle, who is in her fourth year with the horse but just six months at Grand Prix level.
"She has improved our training and the way we train ... I am crediting that time with her for this mark."
Castle's goal is to show the dressage faithful that "you are doing a great job training your horse so you get out there with the adrenalin running to put up a great performance".
It takes four years to enter the Grand Prix level but another three to four to acquire a modicum of "greatness".
In 35 years of dressage, she has won three grand prix titles but yesterday was her first victory here although she had got on the podium at the HOY Show.
So what does the short initiation with Karlos say about the champion combo?
"That he is ticking all the boxes and he's definitely a horse for the future."
Tomorrow's freestyle hopefuls would have had to have achieved 60 per cent or more yesterday to go through.
