No you cannot sell any level castle. That doesn't even make sense to sell your headquarters of main operations in your kingdom.
It depends what real estate company u choose ... But actually no you can't sell the castle at any time.
reeb-bomb-do-not-buy has sold his castle :lol: if that name is wrong search for reeb99 scout his ally Reebbomb or whatever
I'm pretty sure you can't, but I heard if you wall some guy named Swabia, he might be of some use. Good luck! ️