The keystone?
That is correct! Congratulations, your turn! 
keaton, I’m looking forward to yours. 
Here we go:
D I N G * D O N G * D E L L * K I T T E N S * I N * T H E * W E L L
Wrong guesses: R A
Hangperson: HA
I’ll guess an E.
My first guess is a “T”.
Both right. (Is “E” ever wrong?)
I thought E was a safe bet in one that long.
I will now guess an O.
Yep. Updated.
Hmm… not where I expected it.
I will guess an R.
First wrong one! Mwa ha ha!
I forgot to mention it’s a quote, by the way.
Okay, I’ll go back to the vowels and guess an A.
'Fraid not!
AGH! The vowels have failed me. I will go with the safe bet - H.
I’ll try an “N”.
Yes and yes. I get the feeling Lady O might have an inkling…
Is it, “Ding dong bell, kittens in the well?”
Yes! Well, ding dong dell, but I think we can give you that.
Mumm-ra says it in “Locket of Lies”, for those who don’t know. He was starting to lose his menacing aura around this point in the series. From here, it was a slippery slope to “goodie goodie goo drops”.
I wasn’t sure if it was dell or bell.
_ _ _ _ _ * _ _ _ _ _ _ * _ _ * _ _ _ * _ O _ _ _ * O _ * _ _ _ _ _
It’s also an old nursery rhyme and my first guess is “O” Code.