Injury-hit Tigers matched the Betfred Super League leaders - and on this evidence, Cup favourites - for most of the first half, but were undone by Saints’ blistering pace as the visitors took a 12-0 interval lead.
Cas hit back from 18-0 and 24-6 down after the break, but mistakes and penalties at crucial times proved costly and Saints were good value for their 36-18 victory.
They had the game’s best player in full-back Ben Barba, who scored a brilliant hat-trick and looks a certainty to take this year’s Man of Steel award.
Forward Jesse Sene-Lefao was the stand-out for Cas, who certainly weren’t beaten on effort, but need to be more clinical with ball in hand.
Saints ran in two almost identical tries in the opening 40, turning defence into attack in a flash on both occasions.
After 15 minutes Jamie Ellis’ kick to Tigers’ left was picked up by Barber. The Australian quickly handed on to Regan Grace and he raced 95 metres to score.
Danny Richardson, who also had a big game for Saints, converted and then booted a penalty six minutes later after he was obstructed by Grant Millington in front of Castleford’s posts.
Nine minutes before the break Ellis kicked to the same spot. Greg Minikin made the catch and dumped the ball back. Mike McMeeken could not pick up, but Barba did and he scorched the length of the field, though Jy Hitchcox - recalled from a loan spell at Bradford Bulls - chased across and almost got him.
That was unconverted. Saints’ Jon Wilkin got over the line soon after Grace’s try, but Sene-Lefao did well to get under him and prevent the ball being grounded.
Cas defended well near their line and had some pressure, but - without the craft of Man of Steel Luke Gale - could not find a way through Saints’ rearguard.
Saints extended their lead just two minutes into the second period after Cas let the kick-off bounce dead in goal and then turned the ball over inside their own half.
Tigers Captain Michael Shenton, playing out of position at full-back, made an outstanding tackle to keep Dominique Peyroux out, but then Barba jinked through and stretched over the line for a try which video referee Phil Bentham confirmed.
Castleford youngster Jake Trueman came on in the halves midway through the first half - with Paul McShane switching to hooker - and he stepped over for a lovely try, which Ellis improved, just five minutes later.
Saints seemed to have extinguished any prospect of a fightback when Ryan Morgan went over from Barba’s pass soon afterwards and Richardson’s goal made it 24-6.
To their credit, Tigers refused to throw in the towel and Minikin crossed from Shenton’s pass on 58 minutes, Ellis adding a touchline conversion.
It was a 12-point ball game with a quarter of the tie remaining, but Cas conceded a penalty on Saints’ line, Theo Fages broke through the middle and Richardson put on some footwork to go over for a killer try which he also improved.
Barba saved his best for last with a stunning individual try 11 minutes from time, slicing through the defence like a knife through butter. Richardson’s sixth goal completed Saints’ scoring.
In front of a crowd of 5,342, Tigers earned a consolation on the final play through McMeeken’s interception try and Ellis booted the goal after the hooter.