I finally saw the medieval SpongeBob Squarepants episode (entitled "Dunces and Dragons"). All I can say, as an afficianado of both medievalism and SpongeBob, is that Spongebob should have been cancelled at the end of season three. It has lost most of what made it fresh and entertaining.
Basically the only "medieval" joke is ending every verb in "eth," such as in the phrase, "I bloggeth about SpongeBob and expresseth my horror at how bad the episode was." I suppose I could have put the word "joke" in scorn quotes too, since it isn't really funny. Other "jokes" are referring to the king's fool (Squidward) as the king's "doofus," and Plankton's evil laugh ending in a coughing fit.
It may be the worst of all SpongeBob episodes ever (though I still haven't seen all of the fourth season), and it is made even worse by the fact that it runs 30 minutes long rather than the usual 15 minutes. Yes, it is only 30 minutes long, even though it feels like months.
So, in conclusion, SpongeBob Squarepants medieval episode: Not funny. An insult to both SpongeBob and the Middle Ages.