Castillo, 28, has struggled in 2015, hitting .163 in 43 at-bats. It was just two years ago that Castillo hit .274 with a .349 on-base percentage to go with eight home runs and 32 RBI's in 113 games.
It didn't help Castillo that the Cubs traded for Miguel Montero in the off-season and brought in David Ross, making Castillo very expendable. He had been shopped throughout much of the winter following Chicago's additions of Montero and Ross, and has only appeared in 24 games so far this season.
Enter the Seattle Mariners. They have catching problems of their own, with Mike Zunino hitting .179 in 106 at-bats this season. Where it gets worse for the Mariners is with their back-up Jesus Sucre, who has one hit in fifteen at-bats.
In terms of defense, Zunino is very good with his pitch framing, added .88 strikes per game. Castillo isn't great on the pitch-framing end of things, adding an extra .24 strikes per game, but ranks toward the top of catchers in pitch-blocking.
If Castillo can get back on course hitting-wise, to pair with his solid defense, he provides a quality backup for Zunino in Seattle and can play a bulk of the time if an injury occurred. Castillo is only being paid $2.1 million this year and is under arbitration control for the next two seasons, give the Mariners a bit more choice behind the plate going forward.
In return for Castillo, the Cubs received Medina, a 26-year-old right-hander. Medina has pitched with the big-league club for the past three seasons and has been a quality arm out of the pen. In 2014, Medina pitched 57 innings and sported a 2.68 ERA with a 9.47 K/9 rate.
Where Medina has run into trouble is with walks. This season Medina's his walks-per-nine ratio is 5.25 BB/9, up from 4.42 a year ago. Combine that with his K/9 dropping to 6.74 this season and Medina has had his troubles.
It remains to be seen what happens to Jesus Sucre, but the Mariners most likely won't carry three catchers and Zunino and Castillo both have the ability to hit well and play good defense. That leaves Sucre probably being sent back down, as an insurance policy if things go wrong at the big league level. The Cubs were able to get another piece for their bullpen, giving them another option to go to.