Harlequins were routed as three Cas players bagged hat-tricks in a stunning win for Super League's bottom side.
Michael Shenton, Luke Dorn and Richard Owen took advantage of Quins having five players suspended to claim three each out of the Tigers' 13 tries.
After Chris Melling and Stuart Donlan traded early scores, three tries in 10 minutes turned the game Cas's way.
The hat-trick men each scored before Jon Grayshon pulled one back for Quins, but the second half was all Tigers.
Shenton, Ned Catic, Dorn, Owen, Ryan McGoldrick and Shenton helped Cas past the half-century mark.
Further scores by Owen, Michael Wainwright and Dorn completed the rout.
Harlequins coach Brian McDermott:
"We have a lot of young players in there and you cannot leave them in there week-in, week-out.
"These guys need a rest but we do not have the option of doing that.
"It is too easy to accuse people but these guys have to show up every single week and keep putting their bodies on the line and I am proud of them for that."
Castleford coach Terry Matterson:
"Everyone knows we can play some really good attacking football but our defence was also excellent today.
"There is no reason for us to be complacent - you just have to look at the table.
"But we are trying to get off the bottom and if we can continue playing like this then anything's possible."
Harlequins: Melling, Bryan, Sharp, Purdham, O'Callaghan, Hill, Paul, Temata, Randall, Ward, Barker, Grayshon, Haggerty. Replacements: Skee, Heckenberg, Tootill, Mbu.
Castleford: Donlan, Owen, Shenton, Dixon, Wainwright, Dorn, Sherwin, Higgins, Moore, Huby, Guttenbeil, Lupton, McGoldrick. Replacements: Westerman, Henderson, Boyle, Catic.
Referee: B Thaler (Wakefield)
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