And what a missed opportunity.
This is a fairytale accomplishment, a final happy ending after decades of trying to write their own chapter into rugby league's history book.
But it is also a sad tale of rugby league manufacturing its own bad timing.
Classy Cas - a phrase first coined in the 1960s because of their playing style - have had great teams in the past and great players. But they have never won the league.
Four or five years ago they seemed to be a club heading towards oblivion, stopping off at various financial crisis points along the way.
Enter Steve Gill as chief executive and Daryl Powell was appointed as head coach of Castleford in May 2013, and the sat-nav was reset for success.
This year they will win the league - for the very first time - by a mile and with a smile. They will do it by playing the kind of rugby that entertains. A matchday 'down the lane' as the old timers still call the rebranded Mend-a-Hose Jungle, is one of rugby league's most pleasurable experiences. It is a carnival on and off the field.
Yet chances are that few outside Castleford will truly recognise their achievement.
The regular season has now finished - the Tigers finished 10 points clear at the top. They have beaten every one of their opponents in Super League, in most cases twice.
They have often destroyed their opposition, finishing the year with a points difference of +391. To put that into context, runners-up Leeds had a points difference of only +76. They are scoring more than 13 points a game more than their closest rivals.
And yet, they have not been given any silverware yet. The bizarre situation is that Cas will only be mathematically crowned League Leaders a couple of weeks into what is, effectively, a new competition.
The game goes into the Super 8s after next week's break for the Challenge Cup semi-finals. It is a difficult task trying to explain to the non-rugby league fan that this is when the 12-team Super League and Championship clubs are split.
The top eight in Super League play off over seven weeks, but only the top four at the end of that mini league have a chance of reaching the Grand Final.
The bottom four in Super League find themselves up against the top four from the Championship in a prolonged relegation/promotion play-off. This will culminate in a one-off Million Pound Game play-off between the fourth and fifth-placed teams from that mini series to determine who is in which league next year.
But to add to that confusion, in amongst all those games and all the hype of the race for the Grand Final, the League Leaders Shield will be handed out. And no-one outside Castleford will really give a damn about it by then.
It is a travesty that such a big deal will be missed by the sporting world at large. Cas deserve much greater recognition.
Of course, they will get that recognition if they win the Grand Final. But that is not the point. They have already done the hard work, earned their right to be stars of one parade.
Rugby League only has three major domestic trophies to play for - the Challenge Cup, the Super League Trophy (won at the Grand Final) and the League Leaders Shield.
But the League Leaders Shield has been devalued so much since the Grand Final became a thing, that most people in rugby league would not really rank it as one of the majors.
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Winning the Grand Final is seen as the ultimate prize, with some justification. To win it you need to come through a series of games at the end of the year, beating the best of the other clubs and producing your very best on the big stage.
But in devaluing the League Leaders Shield, you devalue the whole Super League season that has gone into shaping the final few weeks of the run-in.
Even the name - the League Leaders Shield - makes the honour seem weak. It sounds like something the kids, the ones not as good as the others, get to encourage them to keep trying.
The champions should be the side who finish the regular season on top. They have proved themselves week in, week out.
Let them lift their trophy, have their celebration and be lauded as the best about. Let's tell their story to the wider world.
Then we can all tighten the seat-belts and ready ourselves for a new competition, the Super 8s with its goal of reaching the Grand Final - a brand surely now strong enough to remain a fierce focus for every team and fan involved.
Rugby league cannot afford to avoid creating a hullaballoo, and what a hullaballoo we should have already been creating about Castleford's achievements.
They're the Leicester City of Super League - a cliche becoming tired among rugby league followers.
But the impact they have made on the sporting world at large, compared to the measure of their achievements on the field, has been minuscule.
And it is rugby league, and the structure and narrative it has created itself, that is as much to blame as anything else.
Castleford will be handed rugby league's League Leaders Shield in just a few weeks' time - what a story!