The Tigers led through debutant James Evans until just before half time, but an effort just before the interval and a try seconds after the break gave the Quins a two-try lead.
And despite the best efforts of last season's bottom club, who grabbed a late consolation through Richard Owen, they never recovered.
Castleford coach Terry Matterson handed debuts to captain Ryan Hudson, Evans, Rangi Chase, Brett Ferres, Stuart Jones and Sione Faumuina, while other new faces Chris Feather, Kirk Netherton and Dean Widders all had to make do with places on the Tigers bench. Brent Sherwin did not make the squad after undergoing ankle surgery in the off-season.
The Quins included Chad Robinson, Luke Williamson and Ben Kaye in their squad.
Prop Daniel Heckenberg missed out with a shoulder injury.
With only two minutes on the clock tempers flared when the Quins drove to within a few yards of the Tigers line.
An all-in brawl followed and hooker Hudson came away with a bloody nose in what proved to be the highlight of a tentative opening period.

The Tigers produced some early handling errors and put the ball to ground on a couple of occasions and the Quins took full advantage, making the early running and dominating possession.
Growing in ascendancy the hosts finally broke the deadlock in the 20th minute.
A good spell of possession and neat grubber forced the Quins to drop out but the Castleford forwards drove the ball back up the field on receipt.
Fast hands saw the ball worked out wide where Joe Westerman sent Evans crashing over with two defenders still hanging on to him.
Kirk Dixon failed to add the extras when left with a difficult conversion.
Chase, making his first Super League appearance, impressed with a couple of half breaks and Westerman saved what looked a certain try with a fine tackle as half time approached.
But the visitors edged ahead in first-half stoppage-time with a converted try.
Some casual passing from the home side allowed Dorn to intercept and race towards the line and touch down under the posts unopposed.
Rob Purdham converted to leave the scores 6-4 at the interval.
Just 56 seconds after the re-start Dorn added another.
At the end of the first set of six, scrum-half Danny Orr kicked through, catching the Tigers defence unaware.
And Dorn had the pace and grounded the ball just short of the dead ball line.
Purdham added the extras to give the Londoners an eight-point cushion.
Castleford rallied but the Quins managed to rebuff a number of promising attacks from the Tigers.
And the Harlequins had an effort ruled out for offside, while centre Matt Gafa was held up as he crossed the line minutes later.
Dorn was denied his hat-trick when the referee pulled him back for offside after he once again intercepted and made for the line.
But Michael Shenton's introduction seemed to spark the Tigers to life and he combined with Owen to give the home side a lifeline.
Good hands from the centre allowed Owen to cross the whitewash five minutes from time.
But Dixon missed the resulting conversion and the Quins held on for a hard-fought victory on the road.
Castleford (4) 8 Tries: Evans, Owen
Harlequins (6) 12 Tries: Dorn 2 Goals: Purdham 2
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