[Oberon]

Well, I'm not sure what you're fishing for here, but Elrond noticed the moon-letters and read them for the Company. They comprised a riddle, to the effect of "Stand at the grey stone when the thrush knocks, and the last light of Durin's day will shine upon the keyhole."

But it was Frodo who deciphered the riddle.

What was the original purpose of the guard tower at Cirith Ungol, and who built it?

[JenB]

Frodo? Wrong book, man...

The guard tower was to keep a watch on Mordor, and it was built by Isildur. What was his brother's name and city?