It was the one with the horse and the king behind some spikes, I could take the king out of there but I didn't know what to do after that, like, I have to put the horse on the other side of the spikes of the exit but after that I don't know...
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Ah! You actually have to use the rook/castle to get the king past the spikes, and the pawn needs to get another use to get things in place. I should definitely have an easier level with the knight/horse beforehand, because having it be introduced ends up implying it's more important to the puzzle than it actually is.