Well, this is the 1300s. Between 1300 and ca. 1330 Ceuta was a battleground between Fez and Granada, changing hands each few years (yep, sounds surrealist but Granada did have her obscure attempts at overseas expansion - they even had a last stroke of fighting over the city in the 1380s). (EDIT: Well, somebody beat me in this)
Meanwhile Granada itself is a vassal of Castile, so in theory if somebody attacks her and she does ask for help, the Castilians *should* come to the rescue. I say *should* because, after all, they didn't move a finger any time Fez did attack the city, sometimes with Aragonese support. And that's despite Aragon had recognized Ceuta as part of the Castilian sphere of influence in a treaty signed, precisely, in 1300.
A Templar takeover is different, though. In the case of Fez Castile would just hope Granada would either retake the city or she could just assume she would take Ceuta and Fez itself some day anyway and go to bed like if it wasn't a big deal.
In my opinion the Castilian reaction is dubious at best and it depends considerably of the circunstances and who's in the throne at that moment. I think there wouldn't be a reaction in most of the cases, and more certainly not after the Trastamaras get the throne in the 1360s - they did lose interest in the southern seas in OTL after all, which gave a free hand to the Portuguese in the area.
However, if it happens in the 1350s, oh well... Peter the Cruel is in the throne. And he doesn't give a crap about fighting anything or anybody. And he is an enthusiast of Granada, the Mediterranean, and naval matters, by the way...