I think you can credit this signing to Andy Hay.
Last year when Aspinwall had being jailed Cas could quite easily have terminated his contract they stood by him. He came back went straight into the team for the CC Cup semi then played the last few games but wasn't offered a new deal. The old coaching staff must have been more impressed by him than the new.
For what its worth I see Aspinwall as the type of player Cas should have kept, he gets the job done works hard and carries with enough strength, a solid player but no more really (I do realise I'm not telling you anything you don't already know). Whilst I would have kept him at Cas I am suprised to see him joining Hull.