For the first time in more than 30 years, the West Yorkshire club secured a league victory at the home of the in-form champions.
It was 1990 when Castleford had previously last prospered in Saints territory and, given how agonisingly close they have come so many times before, it was fitting they produced such a gutsy and spirited display to finally break that run.
It was not until Greg Eden, with his second try of the night, intercepted Lachlan Coote’s pass to race 90m with 99 seconds remaining that Daryl Powell’s side could at last breathe easy.
They had had to defend their line continuously for the last 15 minutes as 12-man Saints – who saw England winger Tommy Makinson red-carded in the 63rd minute – desperately tried to rescue the contest.
However, the exhausted visitors held on to spark wild celebrations among the travelling fans.
It certainly will not make up for last month’s Challenge Cup final loss against Saints at Wembley but will go down as one of the most memorable wins of Powell’s eight-year reign and it breathes life into their bid to make the top-six play-offs in his final campaign.
The excellent Oli Holmes scored Castleford’s other try on a fractious night. Makinson was sent off for a late and high challenge on Niall Evalds in the 63rd minute – barely 60 seconds after his Saints team-mate James Bentley was sin-binned for a swinging arm to the head of a grounded Paul McShane. Castleford’s Pete Mata’utia was also yellow-carded for running into the melee.
Still, Alex Walmsley got 11-man Saints back in it when crashing over from a quick penalty tap in the 67th minute.
Coote converted to leave them 14-10 behind but – after Jesse Sene-Lefao departed for an HIA and with the Tigers line under immense pressure – Sione Mata’utia fumbled after a penalty on the first tackle.
Still, Liam Watts’s ill-conceived offload and then a wild high challenge on Joel Thompson saw Saints apply more pressure until Eden crucially picked off Coote’s pass to deny a certain home score and seal victory for his side.
Just as at Wembley, Castleford led at the break last night.
Eden’s converted try in the 36th minute had given them a narrow 6-4 interval lead.
It was reward for their attack finally clicking, ex-Saints scrum-half Danny Richardson and Evalds linking cleverly down the left to create just enough room for their winger.
Earlier, Castleford had been unable to make the most of such promising positions.
Fortunately for Powell’s side, Saints were similarly profligate, at least until Jack Welsby struck in the 21st minute.
But the visitors generally contained the champions well and continued that at the start of the second period. Although Evalds was bundled into touch after claiming a Coote kick, Jordan Turner did brilliantly to deny Joe Batchelor as the Saints second-row seemed destined to score.
Tigers then forced their own pressure when Derrell Olpherts was just as likely to cross but the hosts’ famous scramble defence came to the rescue once more, just as it did so often at Wembley.
When Bentley was penalised for hitting Gareth O’Brien late, Richardson was offered a simple two points to extend Castleford’s lead in the 50th minute. That would never be enough but just two minutes later, Holmes stood up Coote – of all people – with some splendid footwork before racing 20m untouched to the line for his side’s second try.
Richardson improved and the Tigers were on their way.
Meanwhile, the 2022 Challenge Cup final will be held at the Tottenham Hotspur Stadium before returning to Wembley in 2023.
In another change, the final will revert to a May date in 2022 – Saturday May 28 – and the governing body has also announced a two-year extension to Betfred’s sponsorship of the men’s, women’s and wheelchair cups.
St Helens: Coote; Makinson, Naiqama, Percival, Welsby; Lomax, Dodd; Walmsley, Roby, Leeds, Thompson, Batchelor, S Mata’utia. Substitutes: Bentley, McCarthy-Scarsbrook, Amor, Paasi.
Castleford: Evalds; Olpherts, P Mata’utia, Turner, Eden; O’Brien, Richardson; Matagai, McShane, Millington, Sene-Lefao, Holmes, Smith. Substitutes: Watts, Hepi, Griffin, Blair.
Referee: Ben Thaler (Wakefield).