so - trying to do something with the job priority list...a good move when being new to the game,congrats on that!

sooo...basically the "unfilled jobs" come up,when you have enough people but not enough work to give to them...but that isn´t that bad because when you want to build something you instantly have workers for that - and as you might have noticed how the priority list works:
it takes people from less prioritized jobs to fill the higher priority jobs!

you also asked for a good setting for the priority list so I´ll tell you what I use:
-first up,put your healthcare,deathcare and firefighters when you get them - these will be the 3 ones you should never put down(in my opinion)
-the next spots should go to your basic food production like wheat and fruits,later fish&meat can join there too and after the production also the storage workers for that/the equivalent storage for the food you place there(granaries&produce storages/Fisher huts&fish mongers...you´ll get the idea)
-after your basic needs about health,safety and food are done,we´ll go down to the ressource production and stockpile workers for storage workers...I might put the stockpile workers above the actual production so there will always be ressources that get stored! how you actually put your foresters and miners etc doesn´t matter that much,especially not really in the late game when you´re going for a really long lasting world so just plunck them in...
-after the production is done,you might want some money and you want it flowing all the time...so you put down your tax collectors there
-then I´ll tell you an outstanding move: you might´ve also noticed the ticked boxes in the list - they´re here so you are able to completely shut down a whole arm of production without going through all the production buildings all one by one...so I´m using my defenses(archer towers and ballistas - also inclueding greek fires when using them) as kind of a barrier in the priority list so when under attack I´m able to turn them all on so they gather and defend and later when that´s done I just deactivate them so to save money and - when low on citizen anyways,they´ll go back to everything listed underneath...so practically you will keep running your whole production,storaging,health&deathcare and income when being attacked with that kind of setting...

-and then below you can become creative as you want - put in the rest as you please, there´s no real need to put some things higher than the others...maybe those who handle water like in the noria when you´re watering your fields with a reservoir and maybe put taverns and librarys(or churches if you´re using them) a bit higher...maybe the theatre and the arena workers because they´re there for increasing happiness...anything else...like the military stuff,only use that when us need new troops and have enough jobless people!

anyways - I´m playing this game for a long time now...but I really CAN´T tell,what exactly the castle worker does...put it down and disable him and that´s it...

hope that´s a useful one as a whole - have fun playing! :D

Thanks, that was helpful. :)

maybe a small addition for your "empty manor" - you will also have a count of how many "beds" you have available,meaning how much people your city can take in before no one can join anymore...concluding that the "empty manors" are just not filled with citizen yet