Welcome Drinks My oh my, we've got to get you over to the Tavern - you'll have lots of friends over there.

As to your problem, are you sure it is the same king? If so, did you kill or capture the king in the castle assault? If captured, did you release/ransom the prisoners? I don't think all royals have special strat map pieces, just those in direct line, i.e. princes. There may have been a general married to a princess (or a brother to the king) that took up the crown. If you have a save game from right before the castle assault, maybe you can go looking for other heirs. They would show a crown to the right of their portrait when you right click on their unit.

Edit: Doh ArseClown beat me to it