Volkmann, without significant striking set up, tried early for a body lock and to use the fence to set up an eventual takedown. Castillo, as expected, seemed more fluent and eager to engage striking. Most notably, Castillo's inside leg kicks landed with authority especially given that Castillo is right-handed while Castillo is left-handed. That means their hips were 'open', giving Castillo more distance to wind up and build momentum.
However, Volkmann nearly closed the show with a deep d'arce designed to stuff Castillo's deep underhook after Castillo was taken down from a blast double. Castillo hard to roll back down and change the angle to break Volkmann's grip, which he eventually did. The round was close after that with Castillo trying to achieve his own takedown, but unable to get past the sprawl stalling position.
Castillo started the second using a high-low approach: punching of striking high to give him time to change levels for a takedown. He eventually secured it, but Volkmann was able to stand. The round also featured excellent scrambles with both men trying to achieve dominant position from transition grappling. Volkmann was actually able to secure his own double in the center of the cage and as Castillo reached for a deep half guard sweep, Volkmann again tried for the d'arce. Castillo was able to roll through thereby breaking the grip, but the continuity of the position and defense from Castillo allowed Volkmann to lock up the grip again and again.
Volkmann was the first to strike in the third, so to speak, beautifully timing a blast double in the middle of the Octagon. And what did he do? Go right back to the d'arce. There wasn't much to the round beyond that. Castillo was able to escape a few times, but he could never recover position.
At the time of this writing, this puts the WEC fighters who made their way to the UFC at a 6-8 win/loss record. Castillo performed ably on the defensive end of matters, but lacked the ability to get any meaningful offense going. Also credit is due to Volkmann for keeping the complexion of this fight a grappling contest against a respectable wrestling talent in Castillo. Realistically, Castillo's best route to victory existed in the stand-up portion of the game; a portion that existed for a paltry few moments beyond the first round.