Leek really needed a win to maintain their hopes of nicking a place in the top five, while for Castle, it was merely local pride and final placings to play for.
It started well for Leek when they took the lead after 22 minutes when a good run and cross from full-back Niall Green was headed in at the far post by Lee Cropper.
But on 39 minutes the visitors were back on level terms when Jordan Cole headed in a Dan Cope cross. And two minutes before the break, Castle went in front with a long-range effort from Harry Clayton.
Leek now needed a response if they were to keep their season going late into April, but nine minutes after the restart, the visitors added a third with a second for Clayton.
And it got worse for the hosts as they lost Darren Chadwick to a straight red card in the 73rd minute for a challenge deemed reckless. Ten minutes later and Newcastle made it 4-1 when Clayton completed his hat-trick with a low shot.
And a bad night got even worse for Leek two minutes later when the normally reliable Tim Grice saw his penalty saved by Castle keeper Dave Parton.
The defeat leaves Leek seven points behind fifth-placed Lincoln United with a game in hand and seven left to play.
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