After reading
this thread on the Something Awful forums (it's an excellent tutorial for newbies, and if you're a new player you could do a lot worse than to read it), I swallowed my pride and started a game as Castille.
10 years on and I have taken back Granada from the Moors. My plan is to form Spain. But there's currently a hitch.
Navarra got conquered early on by Sicily. After taking advantage of Sicily's disputed succession with a claim on the throne and a war, I was able to force Sicily into a Personal Union.
The problem is that they were also at war with England. And England is going through and sieging Sicily's provinces. And I don't have any (obvious, easy) way of stopping England from taking over Sicily. England's troops waltz right past my troops and merrily lay siege to the forts. I tried expanding my sphere of influence to cover Sicily, but apparently England is grandfathered in and I don't get a casus belli on 'em.
Not really sure what to do now.