I was extremely impressed with everything he did, it was nearly flawless. He used distance very well, striking variety (tkd, karate, boxing, muay thai), defended the td attempts flawlessly with knees and movement; and that ko, Castillo looked dead, haven't seen anyone look that bad after a ko in years, I was surprised he wasn't out for minutes.
Castillo is one of the better guys at lightweight too, has excellent wins and usually when he loses it's after giving his opponent a lot of trouble (Barboza, Ferguson, Johnson), but Felder just breezed by him.
Reminds me of Magomedov in terms of having a split decision first fight int he UFC that didn't really showcase them, and then (in back to back cards) has a showcase fight against an excellent fighter.