Yeah, killing the horses in that mud was probably even worse than if they'd been able to penetrate the steel armor. Just imagine heaps and heaps of horse corpses, forcing the knights to zig-zag back and forth through the mud, as if the mud didn't restrict their speed in the charge enough anyway... That's actually one of the things that the TW games really ought to implement - movement and formation penalty when moving over ground littered with corpses...
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