Agnatic-cognatic will pass everything to males, females only inherit if there are no available male children. So the highest title would pass to your oldest son, then the rest would be divvied up among your sons, and if you only have one son but multiple daughters the son would get everything.
Unless you want to powergame it, gavelkind is a pain in the ass no matter how you slice it.
He would end up 6/5, yes. And he would have to give one away to remove the penalty for being over his max demesne. And in general it’s good to consolidate holdings (Like Zombo says, high tax provinces are generally what you want ideally but it won’t be an absolutely massive difference).
Just loaded up the learning scenario (Jesus, I didn’t know CK2 had that many tutorials) to see what was up with this Palencia nonsense. Palencia is the name of the county capital of Burgos. Unlike most counties, the capital for Burgos isn’t named Burgos. The city of Burgos exists in Burgos, but castles take the capital spot.
In the event you do start playing as the king of Castile and go over your demesne limit, generally the holding you would want to give away would be a barony, and more specifically non-castle holdings if available (Which you wouldn’t have in this case as both kings only hold castles). Cities tend to provide more gold, churches a mix, and castles the most troops. On the whole having larger levvies is far more important. In this case you’d still want to make sure to give away a barony castle as opposed to a county title/castle, simply because the county title would make the people directly under you in the baronies your vassals and they’d directly be supplying you troops and money, whereas with a barony castle you’re only getting the usefulness out of that single holding itself with nothing else trickling up for you.
Edit: And another tip if you stay with gavelkind succession, ESPECIALLY if you die and everything moves to the king of Castile and you start playing him, try to maintain a single title of your highest layer. In this case, kingdoms. That way the oldest heir at least gets to shove his siblings around and successions are less dramatic (Although with gavelkind there’s more chance for siblings getting ambitious and trying to off you). If you have two or more of your highest power title (Such as the king of Castile suddenly also getting the kingdom of Leon title) then there would be two kingdoms, so two children would inherit large titles and you would lose territory you as the player are directly controlling.
You can abolish titles, you take a fairly reasonable prestige hit (400 for a kingdom it appears) but it would make your successions much easier. Once you reach the emperor title you can have multiple kingdom titles since they would be under the overarching empire, although then you wouldn’t want multiple empires.
This is also highly, HIGHLY recommended for solving heartache with multiple types of succession law among kingdoms. If you find yourself with multiple kingdoms then you’ll quickly realize they can have different succession law, so while you might have agnatic-cognatic gavelkind with one, the other could be absolute-cognatic feudal elective. Instead of attempting to ever so slowly change succession law, you can just abolish that title then re-make which will use the succession laws of your highest title. It costs some gold and prestige (IIRC it costs 400 prestige to abolish it, then you gain 200 prestige when you create it and costs 200-300 gold) but it’s well worth it. And once you’re dealing with kingdoms and empires eating some prestige and gold isn’t too bad.
Edit 2: And as for understanding it, you’ll pick it up by playing. I learned CK2 by watching a few people play it and starting as most honorable newbie Ireland with one county and managed to conquer all the british isles before my kingdom fell apart since it was harder to hold shit together early on. Looking up things on wikis and asking other people who play can help a lot too. Absolutely no idea if the tutorials are worth anything.
